Animation Toolkit in the UK. They have 3mm joints.
At that size you might have to use a wire armature. Joints have edges which is always a problem when you aim for a slim limb. If carefully built, a wire armature can work well enough, while being more flexible, more lightweight, and faster to make.
I have yet to see a well made prefabbed one though. I think you have to make it yourself.
I think you can’t find the answer to your question here, help with writing papers because those who own this information will not share it in chats for free.
I use wire armatures for that size of puppet, 2 x 1.5mm aluminium armature wire for the legs and spine. That works for the arms as well, but you would want something finer for the fingers, like 2 x 1mm wire twisted together. (Actually I use wire most of the time for any size.) Animation Toolkit sounds like the best option for those in the UK. I don't know where in the US to look. I do know a supplier of soft round aluminium wire in the US though, including the finer sizes, and if you do make a jointed armature you will still probably want wire for fingers. Whimsie: http://www.whimsie.com/aluminum%20craft%20wire.html?gclid=CJSi4bjR6...
Ignore the spammer "Jennifer" above, and don't click on any link in "her" post. At best it might actually be about help writing papers (which is irrelevant to this question, and indeed to this board), but it's equally likely to be an attempt to install malware.