Easy. Go to;
http://www.eaacorp.com.
Click on PARTS, 7th tab down on left side of page.
Scroll down to More Accessories.
Click on Parts diagrams with prices, under More Accessories.
Click on MP514K.
IIRC, it's item #40, piston cup.
You'll probably have to call them to order it. That's what I had to do. I bought a .22 cal MP512M from Sportsman's Guide and the seal was collapsed from the factory. I ordered my replacement seal from EAA over the phone. The 512 and 514 use the same seal, IIRC.
They will probably want the serial # of your 514 to ensure getting the right part.