This is a great source for inexpensive break-away collars if you like to change out the colors sometimes. Minimum order is 5. Break Free Collar Sales
As for harnesses, I found the Coastal Lil' Pals step-in harness does not rub my Chi, and he can't back out of it. You do need to adjust the harness pretty snug. I can truly not get more than 2 fingers under the strap behind his armpits.