Marki Microwave has always had one of the widest selections of mixers that I have seen. They happen to have one to meet your requirements. I don't recall the owner's name, but he gave me a lot of help a few years ago with a particularly tough requirement I had for an airborne defense project. The guy did some actual testing for determining spur levels for me prior to ever even buying anything from him.
Look here for M4-0040:http://www.markimicrowave.com/default.asp?ID=169
He's got an M4-0050 that goes 0.5-50 GHz, too.
- Kirt B.