I think most of this forum's members were all in your situation. I just started riding FG about a month ago myself.
Some of your options are:
1a) Buying a wheel set ($150~) online and just putting it on your roadie, and stripping all shifter components.
1b) To add on to the above option, removing some of the chainrings and installing spacers and what not to make it run just right
1c) Even more work but is highly unnecessary unless you really care for asthetics, shearing or filing off any shifter mounts on frame (downtube shifters) and deraileur hangers.
I'd recommend this ONLY if you've got a roadie already and is 90% sure that you'd like a fixie in place of your roadie.
2. Buying a "beater" bike from a guy on Craigslist.
Most fixie riders know their stuff. This means you're (usually) not going to run into sh***y bikes, and hey, if it IS sh***y, it's a beater bike. The upside to this is that you can sell the bike later for about the same price. For eg) I buy a $200 bike, I can probably sell it a year later for $200.
3. Buying a beginner bike from an online dealer
Vilano and Micargi are some bike brands. Not the best ones, but does work. If you go this method, though, you'll probably end up upgrading your bike along the way. These are typically $200~, so you're getting a complete bike for the price of a frame.
4. Buying a bike from a bike shop.
Only if your town has a fixie shop. Small fixie shops know their stuff and sell bikes that are tuned to perfection. The downside is that these typically run $700+
5. Building your own bike.
NOT recommended unless you have lots of bike mechanic experience / have a nearby co-op that you can go to for some hands-on help. I'm assuming you don't have all the tools necessary to build a bike (most people don't) so that's going to cost more to you. The upside is that you can end up with a dream bike, the downside is that it takes time and is often a hella more expensive then your beginner stock bike.
Final thoughts:
I'd say buy from Craigslist or an online dealer for $300- if this is your first bike. Saves work from conversion and generally looks neater, too. Upgrade parts as you see fit. You can sell these later for not much lower then the price you bought em for.
Once you've got some FG riding under your belt and know exactly what you want, (you can only find this out from riding FG) upgrade to a higher end bike.
Or you can just buy a higher end bike outright if you're stinking rich.
PS: If you ARE stinking rich and you're throwing out some parts from your high-end bike.... I accept donations.
