eBay has a good bit of traffic on collector items. I would look there. What kind of condition... That is everything. I have seen stuff like that from $50-100 for "project" motors to $1200-1500 for "like new" restorations ready to mount on an antique boat.
An example, I know about a matched pair of 1961 Johnson 30hp electric start motors just begging to be restored for mounting on an appropriate era wood hulled day cruiser... Motors are for sale, in running condition, cosmetically "ok"... for $700. Non running engines like them that appear to be restoreable are $75-100 each.