I've used sand, glass, oxide, and plastic in my blasting cabinet. Unless its something real sensitive or delicate I personally use find sand that is made and sized specifically for blasting. (no river sand or play sand)
Common since goes a long way. Some practice on a couple scrap pieces will serve you well in adjusting the pressure, the distance from the piece and overall operation.
....while I do all I can in the cabinet I agree with Tim. I take the big pieces to the guys who do it for a living and do not be afraid to emphasize to them you want to be sure to get "straight" pieces back. It may sound tacky but it'll go a long way as they think while they blast.
Oh, I buy my sand from a local brick yard. $4.25 per 50 lb bag. Some concrete mixing places carry the real deal as well.
