Check the 'For Sale forum' to see if anyone is selling one. Odds are good on buying a used engine since rarely do any of these things fail in a major way. As long as it doesn't have any physical damage it's likely you won't have any problem. eBay is a good source as is just Googling. I'd try to get one no older than a 93 as there are several improvements in the later engines. In '93 the rockers got needle bearings, around 97 or so the transmissions were beefed up. Take a look at the Goldwing History page and you'll see some of the changes that were made by year.