There are H3 buffers out there for sale BUT they will not work in a standard carbine buffer tube on your 308.
Armalite carbine buffer tube is 7 5/8" inside depth and H3 buffers work on a 308.
Heavy Buffers.com will build you a buffer heavy and short enough to work in your standard carbine buffer tube.
All that info was in the stuff 98Z posted for you to read.