Betty,
Hello lady! In my previous life, I used MapPoint to help me drive geo-specific promotions in the US by querying, say, a 100 mile radius from my desired location - then I'd add the matched zip codes to my database to use in a multi-select list to help build my segments. I think your source and process is dependent on your use case - my example fits for a local event promotion, but I can also see where a different use case could by met by segmenting by CRM sales territories.
FYI... I think I read that MapPoint was discontinued last year, but I'm sure there's an equivalent online tool these days. I hope that helps!
Best,
Kristin