I mostly look on eBay in the Action Figures - Military and Adventure category, for accessories. Try 1:6 scale clothing. Not surprisingly it's best with military uniforms, seems to be a big market for that. But I've used a suit made for a Winston Churchill figure, and another for Sun Yet-Sen, that are well made and far better than my sewing. Also find chairs and random props, but only occasionally the ones I need, so most things need to be made still.
Barbie dolls and many action figures are 1:6 scale.