http://www.baroquetrumpet.com/intro.htm
Here's a shop that sells them . I have a Tomes trumpet and I think it's very good. Other good instruments are made by Egger, Naumann, and Barclay.
They play much differently than the modern instrument. They punish overblowing, and are not as loud or bright as a modern horn.
http://www.earlybrass.com/nattrump.htm
Here is a good website with lots of info. What do you want to do with the baroque trumpet?