I’ve spent quite a lot of time researching and rooting out the various suppliers of 15mm scale sci-fi miniatures both in the UK and the US. This is as near to an exhaustive list as I can provide. I will add comments on each site over time and probably put up some reviews of products I’ve purchased (though I haven’t bought from all of these sites by any means).
It’s worth noting of course that 15mm scale varies somewhat between sculptors. It’s supposed to mean an average human is 15mm from toes to eyes, but what is considered average varies and some sculptors go with a more ‘heroic’ or exaggerated style. For this reason I generally prefer alien ranges, especially as I intend to use my 15mm miniatures for 12mm scale (1/144 to be precise) gaming in Heavy Gear Blitz, where 15mm humans would look extremely strange!