Help with mags?

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I'm buying a non S7 KSC USP Compact off a friend of mine however he only has one leaky mag

I'm pretty sure I can fix it but I'll be wanting some more mags anyway. Thing is I've looked what seems everywhere for the non System 7 mags and it's like looking for a needle in a haystack

Does anyone know if it's only KSC's mags that fit or if I could get KJW mags or even make it so I can use the new S7 mags to work?

I've also heard that the slides are interchangeable and occasionally the mags but I can't find anything on Compacts and only sketchy info on others.

Any help at all would be greatly appreciated!