Swap meets and occasionally eBay or Craigslist. Also, buy in bulk. Quality sand is unfortunately not cheap. Are you checking with body supply stores that sell a lot of it? They usually can price more competitive than your local hardware store.
Here's just a couple examples. This brand has been the best I've used outside of 3M red or green, but those are like 4 times the cost of these. Ive tried the sticky back longboard rolls for blocking, the hook and loop dustless discs for my vacuum sander and the sticky back 6 inch discs for DA, all of them work great, are consistent, rarely clog up and last a fair amount.
Here's a Canadian company to check out. I have not used this kind yet so I cant speak on performance, and I actually dont even know what it costs this was sent to me in place of a roll of duragold because my last order they were out of one of the kinds I ordered and they asked if this would be ok so I said sure. But it appears to be pretty decent quality