I try and not use full retail places like NAPA because of the expense. I have used them in a pinch but for stuff like this I use these guys. They are re-builder's - at least one of them anyway - and I have been happy with them so far. They are usually backed up and you can wait 30 days or so but they normally have a core to rebuild and then you send them your core after the rebuild arrives. I have only purchased full under the bed 2 and 3 line boosters from them. No idea if they would have a firewall mounted unit but doesn't hurt to ask.
Powerbrakes.com
Power Brake Sales
1517 McCormack Avenue
Sacramento, CA 95811
