Hi,
Safe and Sound by Roxul is a good product too to build something like that. Also, you can find accoustic foam on eBay depending on your needs. Foambymail has a product with the same density than major accoustic brands for a fraction of the price. I have some in my home studio and it does a good job for what I needed.
http://www.foambymail.com
http://www.roxul.com