I bought a 48cc kit off of ebay. It cost me $170 including shipping. The bike, I found on Craigslist for $70.
I did encounter problems. I am not what you'd consider to be a mechanic. but I do enjoy tinkering and learning as I go. My first problem was that the sprocket hole was undersized. I had to look up a machinist through a buddy to enlarge it. I also under tightened the chain tensioner and that caused it to turn into my spokes during a test run. I had to pay to have the rear wheel trued. Now I just need to fine tune the carb a little and I should be good to go. I am looking forward to taking it on a 10 mile or so ride!