The evaporator core is under the dash and requires the removal of the lines at the firewall, and then the removal of the dash to access it. The evaporator rarely fails so I would try getting a used one from a parts car. You could get one from a salvage yard, or from one of this forums members who is parting out a Gen I SHO.
The removal of the dash is time consuming so the $1200 quote is not unusual. The question is do you want to put that kind of money into your car. The job is not all that difficult, but it is "intense" as anyone of the forum members who have done it will tell you. It would probably be a good idea to replace the two A/C lines if you do replace the evaporator core. Then the system has to be vacuumed and recharged.
This job ends up being a "you may as well" job because with each part of the job it leads to something else "you may as well" do also, e.g. convert to R-134a.
So, if you are not afraid to tackle a few nuts and bolts you can do this yourself. rangerj