Where's the best place to buy charms for BJD's(Dollfie's)?

I don't know about charms, but I mostly buy BJD jewelry on Ebay and in the Den of Angels forum Marketplace (you must be a member 25 days and have 40 meaningful posts to gain Marketplace access). I think Volks US has some jewelry.

Re charms, the smallest dangle charms for earrings might work. I do not know where to find them though I would try the bigger craft shops. Human bracelets and necklaces a year or two ago had a style with lots of small dangles. Some people would take those off and use them in BJD jewelry. Also slightly larger earring dangles and flat bracelet charms can be good pendants for BJDs. And I know one person who strung small beads on elastic as BJD bead bracelets. So check craft shops and places that sell cheap jewelry you might take apart (like Riteaid).

If you make necklaces for BJDs, add a simple clasp in the back because people shouldn't pull necklaces over a doll's painted face.

Online -- check any BJD shop for jewelry. You could also try looking at beading sites (like Artbeads.com) and look at their smaller beads. Also try googling things like miniature beads or miniature charms.

Offline -- check any craft store for smaller charms and beads.