TM Desert Eagle Square O ring Magazine leak.

[Shinotama]

Hello peeps, I've been taking a TM desert eagle to service it, guns fine, magazines however are leaking, I've sorted the rest, the input valves are being looked into as ones faulty so I've isolated it as being the top section.

The real issue I'm facing is that I cannot for the life of me find the rectangle/square one that I need to rep lace inside the top section of the magazine, I was wondering if anyone knew that others would work, what size if I was to buy just a circular one etc?

Help? D: