Mazda still sell all engine seals and gaskets for 12a twin distributor and 13B engines. If they dont have the parts in stock the Mazda dealer can order them direct from Mazda Japan. I ordered a couple of sets of seals and gaskets for my even older 10a rotary with no problems at all, I had to wait 2 weeks while they were flown in from Japan.
I recently also had a 121/rx4/rx5 5 speed gearbox rebuilt, I had a local Mazda specialist do the work and he had no problem sourcing parts to renew the gearbox. They normally only require bearings, possibly a new syncro or 2 if its been abused, and some welding and machining to the shift forks if your gearbox is experiencing jammed in gear problems. I doubt (you could try though) that syncros and shift forks are available new, however I know there are at least 2 gearbox companies in Sydney that can custom make obsolete gearbox components. The US probably has 100x that number ! Your best bet would be to find a second gearbox and make 2 from 1 if there is too much wrong with the one you have.
OR if you are using a 13B, use a gearbox out of a Rx7 and have the gear selector housing shortened to suit your car so the stick comes out the factory hole, will also require a custom mount to be made though.