What size blaster and what compressor? If it's a small hand held unit you should be able to get blasting sand from just about any hardware store but if you want soda or something else try TP. TP (not toilet paper) has just about any kind of blasting media or machine you could ever need.
Charlie, use black slag. You can get it at tractor supply or hf. A normal general use pankake can handle a 10lb unit. The black slag works well for rust, and ok for old paint.