new expansion cap needed

Hey all well as some may already kbnow I've had some radiator probs lately so I have another on its way. Today however I squeezed the top radiator pipe and when I did I heard a hiss from the cap of the expansion tank, its leaking air so maybe the caps had it too. I thought these caps were airtight lol. Anyway can anyone point me out in the right direction to get a new cap, albeit on ebay or on the net thanks.

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