Hello Andy, Welcome to the forum...glad you joined us
Do you have a bicycle yet?
Have you decided on a 2-stroke or 4-stroke?
Would you like a rear mount or center mount motor?
If you already have a bicycle, look at this thread to see if your bike will fit a center mount or a belt drive rear mount. If not, most rear mount chain or friction drive will fit most bicycles....http://motorbicycling.com/f3/what-kind-bike-would-you-recommend-510.html
How mechanical are you? The two-stroke takes a little more work and a lot of maintenance. The four strokes cost a whole lot more then a 2-stroke.
You can find a dealer here or check ebay...alot of the dealers also sell on ebay....http://motorbicycling.com/f3/find-dealer-review-1730.html
There is also a few that sell complete bicycles if you want to go that route
If you have any questions feel free to ask...always someone here willing to help out.