Firstly, he did write a relatively small number of pieces for piano. These are mostly early works, and are not generally reflective of the Wagner you might know - but in them do lie some of the seeds. There is a collected edition published by Dover which is reasonably priced, and some of the pieces are available on IMSLP.
Then, there are the various transcriptions of his work. This tends to be largely of his operatic works, and is much more the Wagner you probably know.
The best of them are by Liszt, Tausig, Brassin, Moszkowski and Gould. The Gould is in print and can be purchased through most music shops online; the others are largely on IMSLP, though some are in print as well. The Liszt ones are in a Dover edition, and the Tausig Ride of the Valkyries is published by Schott (available only through the German Amazon site, as far as I can tell).
Most of them are in the difficult to nightmarish range.
Topic: R. Wagner