1985 GMC C1500 leaky tranny

Transmission problem
1985 GMC C1500 V8 Two Wheel Drive Automatic 10.9 miles

Dear sirs, I recently replaced my 350 turbo tranny with a rebuilt one. A buddy of mine took it out and I put it in, I hooked up the kickdown cable and noticed it didnt have a gasket or seal, well it leaks pretty bad. My question is what kind of seal or gasket do I need, and where do I get it. Thank you so much Larry