I have a scout and stormtrooper costumes that I built on my own. Was costly though! In total, over 3000 euros were spent in plastic pieces, accessories, tools, glue, suitcases for transport, etc.
I'm not into Star Trek at all. I guess it takes some networking to know where the conventions are, but it's really not that obscure as long as you can go a comic book store. There's always local events promoted there.