Search Super Chunk or OC703 traps. I bought some for about $100 USD and cut them into 17x17x24" triangles (you'll get 9 per 2" sheet of OC703) and then covered them with speaker cloth from Joanne's Fabric for about $20.00.
Are they all made of the same material? I dont have much money, these looks like a cost efficient deal.
Thanks for moving this to the public forum. You already got the right answer, but I can add a bit more: That foam is not acoustic foam, and the data they show is fraudulent. The data shows it as being very similar to an Auralex LENRD foam corner, which itself is not as good as more standard materials such as rigid fiberglass in a 2x4 foot size. But when I measured that brand of corner foam in a lab it absorbed far less than claimed, as you can see in the graph below. (SOURCE)