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If you dont have any luck here, try doing a classified over on www.yblocksforever.com . One of the members there sold me a damper several months ago. It was excellent. Price was $60, which included shipping.
If you don't have one at all then you would have to pay the core value of about $40.
You need a good one. Buying a used one risks getting a not so good one which is why I spent the money to get one from a reputable guy that provides a warranty.
Go try to buy one for a much newer engine. They're not cheep.
I sent my damper to john mummert and it is 120 dollars with core so about 140 dollars total. It is hard to find a good used one that doesn't have the rubber cracked and separated.