I got the tiles at Amazon.com, tub of 400 for $18. Should be the first hit on a search for "colored plastic tiles" at the site.
They're blank. I painted the A, B,C 1-10 on the red, blue and yellow tiles. I use green tiles for player initiatives, and since characters change, I use a dry erase marker on them, usually just the first two or three letters of the character name. Helps me remember PC names, and I think it keeps the players a little more immersed when their turn comes up and I refer to them as their character.
As for associating initiative tiles with monster tokens (which are just Go beads), I typically match red to the numbered tokens, blue to the lettered tokens, and yellow to "other." If I were to redo all these pieces, I would tweak the system a bit, coordinate the matching a little better, i.e. use red paint to mark 1-10 on the white go beads for one set, use blue paint to mark 1-10 for another set, and so on.
Also, the black beads replace white beads when the monster dies, so everyone knows its a corpse.