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You could contact alphacool directly (they have a replacement on their site but I'm not sure where they ship or where you are located). Or check your local hardware store.
Appreciate the replies. Unfortunately, Aquatuning.us doesn't seem to sell this particular part unless it's incredibly hidden in their category system.
I emailed aquatuning.us support, hoping to hear from them. I'm actually quite pleased to see they sell the Alphacool Copper stickers and front faceplate to this reservoir. lol
aquatuning rep contacted me via email, they're hunting down the gasket and will shoot me a price in a day or so. pretty stoked, i'll be able to use this reservoir again.