I got mine from Leslie's pool supply (they are a chain so they should be all over) It was $12 for 50 lbs. Sand rinsed great in a 5 gal bucket. I did it in 2 batches. Just pushed the garden hose around in the sand while the bucket was tilted. All of the dust and fine particles wash right out. Give it a few minutes and you 'll know when it's done. It will go from murky to cloudy to clear. Total of maybe a half hour from the trunk of my car to the bottom of the tank. Tank was a little hazy for an hour or so after putting it in.
Just remember to keep the filter off when adding the sand and until the particles settle in the tank.