If you like coco choir type substrate there is http://www.reptichip.com/ find a dealer near you or buy it from the show to save money. There are other similar substrates with companies who also have dealers in different area such as http://www.prococo.com/. Advantages you can go as long as 2 months without having to replace it completely.
After that you can use aspen http://www.petfooddirect.com/product...ressed-bedding lab quality, one bail goes a long way.
And after that un-printed newspaper which is often free you you have to do is ask for the end of the rolls at your local newspaper.