You used to be able to buy frames but they seemed to pull back on this a little lately. My 'Mercton' (Flamingo 'Merc' alloy frame with Brompton forks, stem and seatpost) is testament and at the time I built it in 2007 it was easy to get the parts in the uk from SJS cycles or Fudges but times have changed somewhat it doesn't seem so easy.
However - it must in some respect be possible to get parts as spares - otherwise how would your repair your bike after an accident for instance.
Otherwise it's probably best to search for an unloved older base-spec model, strip it, powder coat it and make it your own.
Dealbreaking parts it must have are forks, rear triangle, suspension block long seatpost, folding stem, clamps and chain tensioner. Other stuff is generally customisable though things like derailleurs are pretty involved. Brakes can be updgraded, but if you want to fit short-reach roadie callipers you need to make up drop-bolts as the bikes are very much designed for long reach callipers and mudguards.
Lots of bling parts can be had via bike-fun or brompfication (clamps, suspension upgrades, TI everything) but they are pretty expensive.