Most veggie seed racks in garden centers will have at least one variety (spelled either pak choy or bok choi). If you want more variety, check out agrohaitai.com or evergreenseeds.com. Both sites have several kinds.

Kitazawa Seed also has a variety.

This is an incredibly common vegetable to find seeds of. Just look at any place that sells seeds, Wal-Mart, K-Mart, Target, your local Garden Centers, etc.