Google is a good place to start, typing in "Electric car conversion kit" came up with web sites like:
Electro Automotive: Parts Price List they will sell you all new parts that in theory should bolt together for installing the drive motor, you will often have to have a welding shop fabricate a few new motor mounts for you and a battery box along with modifications to the body/frame of the vehicle, for those modifications a body shop or hot rod shop would be a good place to start.
Batteries can either be ordered online if you want something expensive like Lithium batteries, lead acid batteries are best ordered either from a golf cart shop, auto part store or renewable energy system installer.
Most people on this forum opt for the cheap route, that is finding a used motor, making their own adapter plate or finding someone to make it for them and searching out the best source for each additional part that is needed.