For lower paying positions, a company might use a number of contingency recruiting firms, none of which have an exclusive on filling the position. In this case whichever recruiter brings in the best candidate first gets paid. Everyone else loses. There are also recruiters who, when they find an interesting resume will mail it to several companies hoping to get lucky. These are contingency search firms.
Some companies operate as both contingency and retained.
You can select recruiting firms by their location, however this is mis-leading. A search firm in New Jersey might know of a position in San Diego. The San Diego company may have a parent company in New Jersey, or, the recruiting firm may have done a good job for someone in the San Diego company. Recruiting usually transcends geographic boundaries. However, if you have a specific location where you want to live, i.e. perhaps where you now live, then start with local recruiters, but don't stop there.