As Paul notes, if people become aware of your interest in these old boats, often that will result in someone sharing information that might lead to a canoe. Benson has mentioned several tools that may lead to finding a canoe. The key is to recognize that if you are serious about locating canoes, then you are probably going to be on the search continuously.
One part of this process that is sometimes very complicated is teaching the seller how to reasonably valuate what they are selling. More often than not, sellers will assume that a wooden canoe carries the value of a replacement new build, and they don't. Politely correcting sellers' obviously incorrect assumptions can sometimes result in the seller correcting their valuations and descriptions. I do sometimes respond to obviously incorrect advertisements and "coach" the sellers about how to correct their ads and properly price their items. When I do this, I never mention WCHA.
If the canoe is one I intend to purchase, then a I do as Benson suggests and make an offer and an explanation.
Sometimes I go home with a boat, and sometimes I walk away. Actually, I walk away far more often than not.
I personally think we need to be really careful not to overpay and allow the market to slip beyond realistic expectations. Most wooden canoes are not scarce enough to warrant overpaying for them. And....as many of us know, even if we do every part of the restoration, the end value of a boat is rarely equal to the cost of ownership. It's just that way.