From the humane society: “Acceptable bedding materials for Guinea pigs are wood shavings, shredded paper, processed ground corncobs and commercial pellets. Do not use cedar shavings or sawdust as bedding. They can be harmful to your pet.”
Here is an alternative from ehow: “Fleece is a cost-effective alternative to traditional bedding. The initial cost of fabric and towels might be higher than a bag of shavings, but it’s not a continual expense the way disposable bedding is. Fleece works as bedding because it allows the urine to wick through to an absorbent layer underneath, leaving a dry surface for the guinea pigs to walk on. And, on cleaning day, you just have to bundle it all up and throw it in the wash!”
Other keepers suggest straw or hay.