I also buy my aspen at Walmart, same as young SnakeMaster up there, I buy those two bags at $4.00 a piece and this tides me over well for the week or week and a half, sometimes 2 weeks. (I only have 4 snakes and I mix carefresh and aspen for my bigger snakes)
I bought a big bag of the small mammal aspen bedding from Petco once but opened it up and it smelled slightly like chemicals so I'm not buying that again- but there may be other brands of small animal bedding that you can buy in bulk... (I'll try to remember to check when I get home and give you the name of the brand I bought so you can avoid it)
I'm sure others will be able to give you more suggestions.
Good luck