I've been using Super Store Finder for WordPress for a few years now and highly recommend it.
When I first started using the plugin, I paid the developer for a customisation which tweaked it to my needs but haven't had to have any contact since as it's always worked perfectly!
I've currently got around 3900 stores loaded and it works as well as it did when there were just a few stores in the database.
New features are added from time to time through plugin updates (at no extra cost I should add!). I looked around at a lot of store locator plugins before choosing this one and I'm so pleased I made the right choice.