Howdy Wayne. There is a "swap shop" section for buying and selling motorized bicycle stuff; http://motorbicycling.com/forumdisplay.php?f=23
But not a great idea to depend on em like you would a car, motorcycle or even a moped. The 2 stroke Chinese kits are a joy if you like to tinker. Not at all recommended for commuting unless your really good with trouble shooting and mechanical repair.
The 4 stroke kits are much more reliable but it is still an engine strapped to a bicycle. Great fun and if you're handy, a much better choice.
Before any of that, your trusting your well being to some guy/woman who may have assembled 1 or a hundred and may have no clue. And your on a second hand bike doing 30 MPH.
I would strongly suggest a used moped/scooter. Will be about the same $ but you then have a manufactured form of transportation with lights, horn, suspension and mass produced replacement parts.
Many folks sell their motorized bicycles for around $500. While I think that is to cheap considering how much work goes in to assembling and maintaining, (They all take a lot of tinkering & maintenance) $500 about covers the parts for a decent and safe build.
You also need to be a good bicycle mechanic.
But they are a blast!
Best of luck 2 you and feel free to ask any questions. Some great people here who love to help.