Once again, here's a link https://www.guineapigcages.com/bedding.htm
I've been doing a lot of experimenting and I've settled on kiln dried pine shavings that I get from a local tack and feed store (provides supplies to stables). I get a GIANT bag for $8 that will probably last me 2 months at least.
Carefresh is very nice but also extremely expensive.
Aspen is good and perhaps better for the pigs than pine but is more expensive (still economical compared to carefresh). Get from http://www.ferretstore.com/70cuinlm100a.html
. If you buy two bags at once, you'll get free shipping.
Some people have also made their own reusable fleece/towel cage liners. This is extremely cheap but requires a lot of time doing laundry and you have to initially sew it together. I'm tempted to try it because I'm tired of the pine shavings that are all over my living room floor. I'm just not sure how often I want to be doing laundry.