Lara, there are a few websites from which you can buy music in just about any (brass) instrumentation you can name. I've used (all?) these in the past with excellent results everytime: the individuals/companies seem to be really "up front" and honest in their dealings and their prices aren't all that bad.
For starters, how about Michael Stewart (
http://www.stewmuse.com). Then there's Windmusic Plus
http://windmusicplus.safeshopper.com/8/cat8.htm?103 .
If you are thinking of a trumpet quintet you could do a lot worse than taking a look through the pages at Triplo Press
http://www.triplo.com/index.html . They even have midi files and sample score sheets so you can see what you're getting into.
Eighth Note Publications
http://www.enpmusic.com/ come highly recommended.
I've never used this site but it looks like it might have something interesting.
http://www.worldofbrass.com/
When all else fails I've managed to "read" midi files into either Noteworthy Composer or Finale and then "rearrange" them myself to suit whatever instrumentation is available (with varying results...the only thing I have less experience at than arranging is brain surgery!)
When you are "over the pond" you might find some interesting stuff courtesy of Boosey & Hawkes (although I think they might be a bit more expensive!)
Good luck... going with a "traditional" brass quintet will be (I suspect) a lot easier than trying to find music for a trumpet choir!
ARRRGGG! (banging self in head.. I told I was a lousy brain surgeon)... how could I forget? Jim Fox has music on his website too!