You can go to Walmart and they sell white play sand in the gardening section. I believe it is like $1.67 for a 40 pound bag.

How large is the tank you're filling up? Remember you'll end up having "less" sand because when it is wet it isn't as much as when it is dry. I guess it compacts or something.

I bought 4-5 bags for my 120 gallon and it seemed enough, although I might be getting another bag or two.
To clean it, I basically went outside with buckets and filled the buckets with a bag of sand and let the hose run in it for awhile. Then I would twirl my hands throughout the sand to get it all clean. Then I would pour the water out and repeat about 3-4 times each bucket or until the water became clear. I had like 3 people helping me so it went a lot faster. It usually takes awhile.
Then you can just dump it in your tank (without water) Make sure you put the sand in FIRST.

Then get like a large bowl and place it in the tank and fill up your tank by pouring water slowly into the bowl so it trickles out onto the sand and doesn't swoosh it everywhere. This will keep your water clear for the most part. Then just remove the bowl when you're finished.
I usually do a vacuum everyother day just because I JUST added my fish to the 120 gallon and there are only about 10 fish in it as of now with only 2 bottom feeders and some of the flakes fall to the sandy floor and the fish are so excited exploring their new surroundings I guess they don't have time to clean the sand.

So I have to do it.

But I am getting about 30-50 ghost shrimp today to help me out a bit. Those little buggers pick up anything that is on the sand!
