I'm partial to Golden Gate brand thumbpicks they seem to know how to do it right. They're like National "used to be". National quit making them the "old way" sometime ago and since then they've been unwearable. Unless, of course, I'd like to get plastic surgery done on my thumb to make it fit their new design.

You might try looking online for the maker of your choice and see if you could order a whole slew of 'em through the mail.