If you have a good LFS they carry the part...if not just type "fluval 404 aqau-stop" into yahoo...you'll find a couple of on-line stores that sell the part.
I'd say just make use of it. I have a PROJECT canister filter 1001 and im trying to find the bucket for it to make use of it even though its bout 10yrs old. No point in just leaving it lying around - just use it as extra filtration and pack it with bio media