I used to get mine at a place called Material and Concrete, they sold primarily brick, concrete block and other similar supplies. Or at least I did until they went out of business. Like Achipmunk said, alot of material places have sandblasting media. Also, you might check out the farm stores in your area. Around here, both the local store (Big R) and TSC have blasting media (Black Diamond) relatively cheap, around the $6-7 range.
From what I've seen, playsand or builders sand is just not as efficient as regular blasting sand. It's also more expensive, or at least here it is.
