If you didn't ruin the stock cork /rubber gasket, you can reuse it. I can't guarantee that it won't leak. Wipe down all the sealing surfaces (and gasket) with acetone to remove the oil residue.
Get a new gasket; the stiff fiber type. It seals better than cork/rubber and is reusable. The best place to get one is from a trans shop or a GM dealer. Transmission shops usually stock them in bulk. I think GM sells replacement gaskets indivdually and uses fiber instead of cork/rubber, now. More than likely, you would have to order and wait from your local dealer.
The cork / rubber gasket is what you are calling cork. It is a mixture of ground cork and rubber particles that are bonded together.