Unfortunatley the front seat belts are different on 3 Doors as they attach to a rail at the bottom so you can slide the seat belt out the way when letting people into the back.

I ended up taking mine apart and repairing the ratchet on mine when it just locked up all the time as I couldn't get a replacement.

Might be worth tring some of the breakers see if they have any but if you need it in a rush you could try local scrap yards for a 3 door sierra but they are probably rarer than 3 Door Cosworths.
Rich