I could be wrong, but I don't think Wagner ever wrote anything for the organ. The concept of an "organ piece" seems sort of anti-Wagnerian to me, in fact. Most of his works, especially his mid to late works (and all of his major works) are programmatic. He wrote a C major symphony as a young Beethovenian, and a handful of early piano sonatas, but nothing of any real note, in my opinion.
His best and most famous purely orchestral work is probably the Siegfried Idyll (which of course is recommended), but most of my favorite Wagner pieces are orchestral excerpts from the operas. There are a lot of Wagner Overtures & Preludes CDs out there: I'd recommend the ones to Tristan, Tannhauser, Rienzi, Parsifal, Meistersinger and Lohengrin especially to start out. The preludes in Der Ring probably beat all these, but they're harder to isolate since they go right into the singing.