If you want something very portable but also very hard then get a couple of lacrosse balls, and then if you still want something more akin to a foam roller then nip down to your local plumbing supplier and get them to cut you a length of 4" PVC pipe. Not much beats the lacrosse balls though.
“Begin at the beginning,” the King said gravely, “and go on till you come to the end. Then stop.”
I just got into foam rolling in the last couple of months. At first, I thought it was so odd and that I looked funny doing it. Now, I think it's awesome and while I'm still getting used to it, I think it really does make a difference in my post-workout stretching.