The car I'm interested in shows no keys — where do I actually get a replacement?

A dealership for that make is the most reliable source, especially for vehicles with programmed key fobs or immobilizer chips, since they can cut and program a key using the VIN. Independent locksmiths who specialize in automotive keys can often do the same for less. For very basic mechanical-key vehicles, a standard locksmith can sometimes cut a key by lock code or by re-keying the ignition entirely.