Master cylinder cap gasket...?

I believe I have some weeping coming from the master cylinder cap gasket, and I would like to replace it. I have heard that it is available from the dealer with the cap, but it is also available through the Help! (Dorman) line of products. Does anyone know the part number for the Help! one? Their website fails to list one for our application. Any help is a huge help!
Thanks

where can I even get this part?

It comes with a new mastercylinder cap. I don't think it's available seperately unless you get one from a pick a part or something.