Make it yourself. LRS 'might' have them but I don't know. If you have an old set of sockets lying around, use a wire brush (or wheel) to remove the chromium coating and install that. Just use one that fits fairly snug inside of the core's inlet hose. A 3/8s socket works well.
The original flow restrictor used on 95 & prior year Mustang was discontinued by Ford so all we're left with is the 96-04 version. It takes a little patience to install, but it does do the job nicely.
im trying to remove my heater core as we speak with out pulling the AC, but i cant get the heate box pulled out t get the 2 rear Screws that holds the cover. Ive read that many peple have done it before. Ive removed the 2 nuts on the fire wall, and the 3 bolts on the front of the box. I still cant get the box pulled out to get to the rear Screws. am i missing anything?
its way easier if u disconnect the ac lines to pull it out...i cant see any other way of getting to the back screws if u dont...im sure people have done it without touching the ac but mine wasnt charged so i just disconnected the ac and pulled the whole box off the firewall