I use the Quickrete Premium Playsand from home depot. I got it for $3.48 for a 50lb bag. Like the others said, it is way more than you need but it will still be cheaper than buying say, a 10lb bag at a LFS
. What you don't use just throw it away or dump it in the woods behind your house. What I did was fill the left over from my 10g, which was probably 40lbs or so, into 5 gal
buckets with lids. That way it is kept safe and clean and I can add more sand as needed.
Edit: I have no problems cleaning my tank. Simply wave the vacuum around about 2inches above the sand and all the gunk gets sucked up. The sand will get sucked up but it never makes it to the hose part, it just floats around inside the vacuum part and falls back down.