You can buy sand from a fish store, but a much cheaper way is to get a bag of play sand from a store like Lowes or Home Depot. They sell 50 lb bags for less than $5. You have to rinse it really, really well before you can put it in your tank without it clouding up the water, but I think it's worth it.
Also keep in mind that with sand, you'll need to keep it stirred so that it doesn't form dead spots. You can stir it manually with a plastic fork or something when you do your water changes. Your loaches will help you keep it stirred some, as well. You can also get some Malaysian trumpet snails which will burrow through the sand and stir it for you.