I presume you want a 6-coupled chassis for a scratchbuild project....?
If so, you could pick up a tatty U class for anything from £140-£160 if you're lucky, though it will likely be a very old one.
Another option would be a single power unit from a Mallett, they are very occasionally available as spares.
Phil's suggestion of a 2-6-2 is a good one too.
Modell-Land did have some complete old-style U class chassis assemblies for sale a while back, complete with all the motion, for around eighty quid - sadly they didn't appear to have many, and now they seem to be all sold. I should have grabbed a couple when I had the chance....
Jon.