If you're looking for a round silver bead in 18 or 20 mm, try this place, item BSA064 is the 20 mm and BSA063 is 18 mm. I didn't go looking for chain but they should also have quite the selection.

Looks as if the holes in these beads are quite small, more for beading wire than for leather strip. If you are desperate, perhaps you can try drilling them larger? But if you need "hollow" vs. a drilled bead, best of luck!

http://www.genbead.com/