Room Treatment Options?...What to use?

I am having some trouble with boomy base. No, it's not the speaker placement. I have consulted the Cardas sight and have the Robert Harley book, so I think I am ok there. But the base still seems boomy. I need a bigger and more silent room but don't have the $250K down payment money in the Silicon Valley. I can only spend a couple of hundred dollars. How can I get my hands on some effective room treatment/sound absorption material?...Are there places I can buy this stuff and assemble it myself?..Your help will be appreciated