E-bay/Gumtree is an option, search Thorn bikes often. I have seen some frames there. Or, if really really stuck, Dawes Galaxy or Ridgeback Explorer if you can't find a Thorn anywhere. Again harder to get smaller frames.
SJS also sells frames, and sometimes sells second hand frames at a very good price. (with build up I thought you mean build the whole bike...gumtree/e-bay are your friends for second hand parts such as gearing, stems, shifters, seat posts etc, chainreaction/wiggle etc for things like saddles)
I found smaller frames are harder to get second hand, 5.5" myself, my XTC is a tad long with 55 cm effective top tube length. With a shorter stem for drop bars (what I did) or going to straight bars/sitting more upright (my straight bar bikes all 54 cm top tube) you can work it.
So, a too long bike can be modified by changing bars if you have to, but then you also need shifters etc...if in a rush, a hybrid commuter, mountainbike etc. can do the job unless you must have low rider bosses OR the ride specifies what bike to use.
Happy hunting!
