More generally, what are my options for obtaining a key when a vehicle doesn't come with one?

You have three main paths: a franchise dealership for that brand (most expensive but most reliable, especially for modern fobs), an automotive locksmith (often cheaper, especially for older or simpler vehicles), or, for very basic vehicles, re-keying the ignition and door locks entirely with a new matched set. Costs and turnaround time vary significantly by make, model, and key technology, so it's worth a quick call to a local locksmith before bidding if key replacement is a dealbreaker for you.