I honestly think this stuff has always been available, finding it has always been the hard part. My friend and I used to 'troll' pawnshops back in the day, we used to find some great bargains on 35mm/medium format, rangefinders, lenses and accessories and local camera stores used to (and still do) sell used stuff.
With the advent of the internet came forums and ebay, which made it easier to find gear which wasn't local, or from a seller who didn't want to sell it at a loss to a camera store or pawn shop or what have you.
Cl made this easier yet again as its basically free if you have a computer and can get on the internet. I find this stuff goes in waves, and with newer digital really starting to equal the low/mid end of film quality I think its just more obvious that this gear is readily out there if you know what you are looking for. In NY/Chicago where i've kind of half assed kept track of these things, good deals on MF gear have always existed if you have known what to look for.
I'm personally trying to decide if MF digital will ever be what I consider affordable (it is if I want to do some serious hunting and buy gear a few generations old) as i really do miss shooting rangefinders and MF, but right now cost is still an issue as I don't want to deal with film.