CA18ET Manual Gearbox
 

How hard are they to come by and how much do they usually go for?

I am obviously a fair distance from you, but i have one in my back garden, has been there for some time, it was taken out for a refurbed one to be put in, it needs and overhaul. if you want a good one, then sorry i cant help you. :D

or just upgrade to an s13 box bolts straight up.

isnt the gear lever slighty further back?