I don't know about your area. Here in Virginia, I have bought it at construction supply businesses. I usually only pay about $10.00 or less for a bag. We also have a place here called "Northern Handyman"(used to be Northern Hydraulics). They sell it by the bag. If the sand that you are talking about is just regular sand, like at the beach, I would not use that. It is too course.
I have been buying blasting sand for the last 10 years and have never paid more than $4.47 per 100# bag!!! Home Depot carried it for a while, but their quality was too inconsistent. Cruise the yellow pages. Monument companies use lots of sand; call them and find out their source and get their price while you are at it.