Sarah, you could leave them bare as I'm sure you will find they are so much easier to clean.
We have sand in our tanks apart from our little tanks where we have 20 axolotl babies - and find it so much easier having bare tanks for feeding and cleaning that if I was able to persuade my husband I would clear the sand from them all or permanently silicon the sand to the floor to make it easy for feeding worms and stuff, so they don't bury within the substrate
Petshops are too expensive!
If you are still keen on getting sand; try landscaping/hardware/garden places like Williams/Bunnings/Mitre 10/Ablaze and just ask for playsand - usually costs between $5-$9 for a huge sack/bagful.
Also, we have about 1-2cm height of sand. If you have too much sand you;ll get pockets of gas building up. Our sand we regularly stir/run a turkey baster through in rows and crisscross or use the siphon on a to do same to avoid this.
(Message edited by kapo on April 08, 2007)