surfboard foam (repeated question, sorry!)

[didj]

I have found a couple threads talkng about the foam used in surfboard shaping- polyurethane eps etc.
I have found sites like homeblown which you can buy blanks from but are these non shaped? becasue 50 squid (shortboard) dose seem quite alot for a sheet of foam. I wanted to make it my self as its for a colllege exhitbition (my final project) it is not going to be perfect and i will probably never ride it so i wanted the foam to be as cheap as possible.

I have found a company who supply foam but when I asked if they do polyurethane they asked for the grade,how many and thickness so Im in a pickle as I dont have any shortboard templates (help on this would be greatly appericated) I cant ask the guy what I want, but would it be just as easy to buy a blank from homeblown? also what is the grade of foam (im such a n00b at this ha)

sorry for the wall of text

many thanks
Chris