ummmm.. thrift store if you want to go the hipster 80s conversion route, or thebikebiz.com has some decent framesets for cheap. Honestly, converting old bikes may be the cool hip thing to do, but if you want a nice ride that wont get shot to shit in a few months but is still cheap get one of these
http://www.bikesdirect.com/products/motobecane/messenger.htm, its 350 with free shipping, not the lightest but still nice, 48:16, which is kinda large at first but you can get used to that ratio or just buy a 17 or 18 cog for 15 bucks more. or get a fuji track for 50 more off thebikebiz.com, or if you want something higher end but harsher of a ride, I can recommend some track bikes for around a grand. If you're set on converting, be forewarned that getting the alignment right can be a pain in the ass, so have alot of different spacer rings ready to fuck around with, and make sure you have long horizontal dropouts or else buy a half-link to get the tension right.
as for the trend? I love riding past some kid who has clearly never ridden a fixed gear before, let alone a road style frame of any kind, and for some reason he feels the need to be a bad ass so he is riding brakeless, on a frame thats way to big, with fucking platform pedals instead of clipless or even toe clips. and sees a red light, and doesn't know how to stop so he is wobbling all over the place trying to slow down.
When you convert yours, be sure to leave at least one brake on until you are very very confident in your riding ability.
Also, messenger bags are for faggots and sailors. Basically if you aren't a messenger, then use a fucking backpack, its so much more comfortable, and you won't look like a trend-whoring hipster bitch. oh and riding without lights at night is totally fine until you get nailed with a fifty dollar ticket, so save yourself the hassle and just buy a red and white knogfrog, they are 9 or 10 bucks each and put you on the legal, if not quite actually useful, side of the light laws.
