Which air cleaner housing are you looking for? The very early one from the later plunger bikes or the later one all the bikes used? I think the change was made in '56, but someone else here might know. I assume you are looking for the early one as the later ones are a dime a dozen.
I figured you were looking for the early one. The later one shows up in the '56 parts books, but I think both were used that transition year. Nice early ones are indeed hard to find. I was lucky that my '55 came with an excellent original one.