Depending on where you're located there's several workable options for green/blue screen backgrounds. If you're in the US I'd take a look at B&H Photo Video, Adorama, or Filmtools. They all carry a variety of background types, fabric (from cheap muslin up to really nice foam/fabric backed matte finish background materials), paint, or paper.
If you want to go the fabric route the brands that I've used that have had nice color/luminance accuracy (which helps immensely when keying in post) have been Westcott, Lastolite, and Impact. For fabric there's the option of either plain fabric (usually muslin) or a collapsible background. I like having both on hand. The collapsible ones with the frame are super easy to clamp to a light stand or two for smaller setups/framing, and the plain fabric is good if you need a large background. Price wise the collapsible backgrounds run about $50-75 for a 5'x7' and plain muslin is usually $50-75 for 9x10 or sometimes larger. I would avoid the really cheap fabrics on Amazon and Ebay since they usually aren't specifically a chroma key green and instead just a similar generic green which, without getting into the effect of varying luminance values, will cause a lot more headaches while keying in post than they're worth.
Some fabric options:
Another route you can go is to pick up chroma key paint. The brand Rosco carries a really nice chrome key green and blue as well as digital green and blue. The difference between digital green/blue and chroma key green/blue are pretty minor and either works well in the studio. Green screen paint is really nice because you can also use it to paint rigs or set pieces that you want to key out as well, and offers options for making whatever size background you need. With a decent primer even the quart size chroma key paint covers a fairly large surface, easily a couple 2'x4' or 4'x8' sheets of plywood. A professional green screen paint is about $30 per quart (1L), or you can pick up paint from a big box homegoods/hardware store. If you prefer the DIY paint option I believe that the best color for green screens was Behr's "Sparkling Apple" #S-G-430. Depending on how well lit your background, what camera you shoot on, and what your foreground elements are, you might find the Big Box paint options are trickier to key than a specifically formulated chroma key/digital green paint.
Rosco Chroma Key Green 1qt:
Home Depot Behr Sparkling Apple:
The other option is to use a paper background. Paper backgrounds are fairly common in the photography world, and a decent and inexpensive option for animation too. There's a couple brands that know of that make chroma key green paper backgrounds, the one I'm most familiar with is Savage. They offer several size rolls, from as narrow as 24" to as wide as ~100" and they typically run I think ~18-36' to a roll. Depending on the size of your setup I'd recommend their 53"x18' roll which runs about $20. https://www.bhphotovideo.com/c/product/1508562-REG/savage_46_5318_b...
I use a chroma key green Moulton (brushed cotton) fabric, a brushed cotton, that comes in 3 metre wide rolls. I buy it by the metre from a theatrical supplier. (In Australia, so name of supplier is not that useful to you, but you can get it in the US and probably cheaper.) It is also called Rokel or Duvetyne. I have also painted some card with matt green or blue paint. Basically like a matt finish acrylic interior wall paint. Sometimes a regular sewing type fabric store will have something that is a good enough shade of green or blue, but their cloth tends to come in 1 metre or 1 yard wide rolls so best for a smaller background like 6 ft x 3 ft. But it might be a cheaper option.
The important thing is getting a fairly even lighting on the green screen, and have it far away enough the puppet does not cast shadows on it.
When I put the same green fabric on the top of a platform to stand the puppet on, it catches more light so it comes up much lighter. I took the fabric off and painted the platform with a darker green that came up looking near enough the same to the backdrop on camera, and that made it easier to key. For my own films I rarely make the ground green, I prefer to have a ground the puppet can cast shadows on, and key that along with the puppet over the background image.
The one thing I used once that gave me problems was some green vinyl that was laying around so didn't cost anything - colour was perfect but it was sort of semi-glossy and I got hot spots from light reflecting off it. A polarising filter on the camera lens fixed that.
As I was perusing the site and saw your question, I thought I'd get Nick (my camera guy) to add to the discussion:
If what you mean is where can I find a background after I have it keyed out.
If you aren't a skill matt painter, and don't want to spend a lot of money buying anything this is our home remedy for making backgrounds.
There is lots of stock photos for sale and free but its a gamble to get the correct angle/position.
To composite your own you need 2 things probably. A sky and a landscape in the distance.
I recommond generating your own sky, and maybe combining it with some photoshopped land. That is what I am experimenting with. I started with Blender and the tutorial below it is all free.
I am experimenting with this and combining it with actual photos of our sets, that I map onto a plane in 3D space, I roughly recreate the shape of the set 3D model and place all these elements in Blender as a little virtual Stop motion set. Its hard to setup, but gives you greater control and consitensy over your backgrounds
If you have time/power and are lazy. I use this software quickly turn real elements into 3d elements using a simple camera. You can use this to lazily import real things into a 3d software and place it in space. Just use this to get a model shape. Then take a single photo from your scene's desired angle and project back onto the model of the object. I hoping to use this for mountains in the background
I am striving for as far as technique to make your own dynamic background that you can control.
-Nick the camera Guy