i got my aluminum one from summit & i can't remember exactly, but i think it was a 15 gallon because the height (depth) was the same as my stock one so you can use a hole saw & put the stock unit in it.
http://store.summitracing.com/default.asp?Ntt=fuel+cells&searchinresults=false&Ntk=KeywordSearch&DDS=1&N=115&target=egnsearch.asp
i think mine was only about $180 or so...
good luck