Beer engine seal kit

Wanted to get the seal kit from NB, but it is out of stock.

http://www.northernbrewer.com/shop/brew ... r-seal-kit

I see a few sites in the UK that could help, but does anyone know where I can get the piston seal like below:

It’s the one on the right in the middle. It appears to be kinda spring loaded. Anyone know what the dimensions are?