El Cortez has very few well worn 50¢ chips on certain tables but I doubt you could get a rack of them. The only way I've had luck is to put a dollar down and ask for 2, even then the dealer ask for approval from the pit boss.

CasaBlanca and Virgin River in Mesquite use them but I've heard they're pretty stingy about selling them.

I believe your best bet is as David suggested with Longhorn on Boulder Highway.

You could also try Railroad Pass in Henderson.