I have quite a few bookmarks for domino art info and dominoes, so here goes! I saved these locations primarily for the purchase of blank and predrilled pieces.
Pennywise Arts
Outside the Margins
Your ATC store
Sunday International
Rubber Nation Crafts
Annie Howe's Etsy shop -- mailing list option for when it's reopened
Enchanted Gallery resource
The last link is very helpful. I have purchased from Annie Howe and Sunday International.
I've been wanting to get back into domino art myself - it's lots of fun to work with them. Many people will drill their own dominoes. The ones I have made are not drilled at all. I will use silicone cement to attach the appropriate piece directly to the domino.
here are a few samples in my gallery :^)
Pendants
Magnets
picture frame
All of these are made simply with regular game dominoes found in discount stores.
HTH - have fun!