The Klindworth Wagner I mentioned is long out of print (since 1899) so you'll have to rely on second-hand vendors or libraries, except for Die Walküre which was reprinted by Könemann Musik a few years ago and is still to be found in shops - grey and blue cover. It's very cheap too!
There's no perfect piano score of Tristan, though there is a ridiculous number of different transcriptions in existence. The most interesting historically is by Hans von Bülow, which was published by Brietkopf shortly after the opera was written, but musically it's disappointing. Breitkopf subsequently published two, or possibly even three, other vocal scores including one by Otto Singer who's very reliable, though I've never got my hands on a copy. The usual recommendation is the Peters edition, done partly by Felix Mottl and the rest by Gustav Kogel, which is frankly pretty good.
With the Tristan Prelude I can't really help as I've almost never played it, but I've done the Liebestod so many times (for singers in competitions, mostly) that I ended up making my own version, which I'll post here some time when I've the energy to scan it. I realised years ago that the Liszt concert transcription is usable as an accompaniment since the voice is basically never melodic except when it's doubled in the orchestra, but in places the Liszt is over the top for that duty, so I did a scissors and glue job between Liszt, Kogel and Bülow which works reasonably well.
By the way, that Wagner/Busoni you mention was disowned by Busoni later in life. He did it when quite young, probably for the money, and later took a passionate displike to Wagner as both man and musician.
Topic: What is your favorite piano transcription?