Try
http://www.mcmaster.com/
Search under "Coupling".
1. Scroll down and click on "Chain Couplers".
If you look under Mutli-Flex couplings, these would work. The hubs connect to the shaft. The bad part is that the 3/8" hubs don't have keyed shafts. Another bad part is they only have hubs made out of cast iron. You could try to get the smallest possible hub size, but still it's gonna weight quite a bit...
2. Scroll down and click on "Spider Couplings".
These have lightweight aluminum hubs available, however, they aren't keyed again.
3. Scroll down and click on "Rigid Couplings".
You'll see a nice table format of all the available types of couplings. Do a search for them and see what you find.
If you have the funds, go ahead and check them out. Otherwise, if a keyed shaft is important, you might want to pass...
*I did a search through all the coupling types, and none have a 3/8" keyed hub. Either larger bore size or you'll have to use set screws.