I prefer the ebay kits. I usually buy from a seller called boygoesfast (http://www.ebay.com/usr/boygoesfast
), but he seems to be unavailable at the moment. I would suggest doing an ebay search for"motorized bicycles" and find a kit from a trusted seller with a high rating. The law varries depending on the state you live in. Try searching for your local motorized bicycle laws on google. eg: "South Carolina motorized bicycle laws".
In my state they require a front light, a rear reflector and/or a rear light, and at least one mirror. There also restrictions on how much power it can have.
Hope this helps.