Vacuum nipple

have you tried looking at a hardware or home improvement store in the fittings area. some of them carry brass fittings with nipples...

You need something like this, but with a smaller barb diameter:

http://www.homedepot.com/p/Watts-3-8-in-x-1-4-in-Brass-Barb-x-MIP-Adapter-LF-A293/202254880