There's many vendors on Ebay that offer exactly what you're asking about, either ready-made or custom fabricated.
If Ebay isn't your cup of tea, there are lots of vendors online like the Antenna Farm which offer these things. Many of them have ads on qrz.com. You may find some guys in the forums on qrz.com who offer these services, too.
You could also do some google or yahoo searching...
You have to be cautious where you buy your cable as some out there carry cheap knockoffs which will give you problems.
I have purchased many cables from DPD Digital and they are always high quality, fast service, and very reasonable. If you don't see a listing for exactly what you need they will custom make it.