I looked for a long time and never found them. I pretty well gave up. Then, I got three by chance from a friend. She had bought a $150 darkroom setup advertised in our local Craig's List classifieds, and the 4x5 trays were included. I got so excited that she just gave them to me.
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."
I develop two sheets of 6.5x9 at a time in a Jobo 1520 type tank. Remove the reels from the tank, toss aside. Put the center fill-post back into the tank. While supporting the fill-post, insert one sheet of film into the tank with the emulsion side facing away from the fill-post. Insert another sheet into the tank on the opposite side of the fill-post in the same manner. Cap and fill your tank to the top of the fill-post with developer. Develop as normal.
"Lo único de lo que el mundo no se cansará nunca es de exageración." Salvador Dalí
The Jobo works best for me. The Nikkor tank should work well if you can find one. I used one a few times many years ago, and I was quite happy with it. I have used the Yankee tank many times, but I've been getting inconsistent, splotchy results sometimes with 2X3 sheets. I haven't tried the Combiplan tank, but it also takes that film size.