Reddit mentions of K-Tool International (KTI44910) 10mm X 12mm Flare Nut Wrench

Your mechanic is almost certainly wrong. It's the slave cylinder, very common in Miatas.

Any idiot can do it for <$50. I actually just took mine off yesterday to regrease the pushrod (chirp, chirp). The first time I did this, it was on an NA with no jack in about 30 minutes. It's 2 bolts and wrestling with the stupid clutch hardline. (For the more mechanical: replacing that stupid coil assembly with a SS braided line makes life so much easier for many different things.)

Buy a slave, a 10mm FLARE NUT WRENCH for the hydraulic line, and some brake fluid.

Read this: http://www.miata.net/garage/slavecyl.html

If you have no sockets, you'll need to borrow a socket wrench, some extensions, and a short 12mm socket. The top mount bolt can be had easily with lots of extensions, the bottom requires the short socket and perhaps a little torquing from above. The hydraulic fitting will be on there pretty tight, use the right tool (flare nut wrench). Bleed the hell out of the system when you're done, instructions are in the Miata.net link.

95% chance of solving your problem, and worst case: you spent the equivalent of a decent datenight on expanding your personal skills.