Well you pretty much answered your own question. Due to the fact that there is no demand (and one person looking = no demand in this situation), there is also no supply. For years it has been very limited in the Subaru aftermarket community, because there was a very, very limited market. I own one of about 15,000 <'02 Impreza RS', and in terms of aftermarket, that's just not enough to warrant the R&D.
So basically you'll have to design and manufacture your own cowl induction hood, or pay somebody to do so for you.