Amazon. No fuss, no bother. Always got a great selection either direct or through sellers.
Buy one with a plastic comb when you're starting out. You will get it soaking wet, and wooden ones tend to swell with the...er...moisture
You want one in the key of C to begin with.
My favourites are Hohner. Hohner Marine Band (recommended, eventually) has a wooden comb, the Big River or Blues Band a plastic one. Blues Band (not Blues Harp) is the absolute minimum you should think about, but I'd go a Big River. They're about 20 quid.
Avoid chromatic harps - the ones with the button on the side.