Theres a few models out but most of them wont be available on PAYG such as the 9700 Bold2, 9000 BOLD, Storm 1 and Storm2, 8900 Curve, 8520 Curve and also the pearls if you can find them. I know that tmobile at one point were offering the pearl on a PAYG tariff ie you paid £150 upfront for the phone and one yrs line rental or something similar.
Have a look around at the networks and see what they can do for u regarding a BB.
Best BB's right now are the Bold lineup ie 9700 and 9000 but good luck on trying to get those at a cheap price...the curve range 8900 and 8520 are good entry level BB's then theres the vodafone exclusives the Storm 1 and Storm 2...id stay away from the Storm1 as its not really that good...the S2 is much better i find.
I have the 9700, 9000 and Storm2...all great phones but cost me a pretty penny to say the least....your better off going with a contract honestly if you want the higher end BB's....u will get a much better deal via the networks that way. Other than that you can buy one offline and stick a payg sim in it but it wont work unless you add a BIS plan which will cost you £5 extra.