What kind of project have you got in mind that you are collecting the bottles for?
Someone a few years ago posted a picture of a mini bottle tree (tabletop size) with different color miniature bottles, that was pretty cool, and then more recently there was someone who made a fairy garden with a little bottle tree (and a button tree, that was cool, too).
I'm sure there must be other sources but the only other thing I can think of right now is I bought some essential oils that came in tiny blue bottles.

I have a small backyard garden of normal-sized blue glass bottles. I mostly stick them on cut-off thin bamboo stalks. Smaller ones, I mostly use in my decorative stones. They needn't even be blue, practically any small bottle looks good in a mosaic project.