It's a few more dollars than you might want to spend, but you could change the hubs to the newer version with lug nuts. Eliminate the problem of a spare as you could buy them anywhere. If you could find a used trailer that had good hubs, change the bearings so that you knew they were right, and get your new wheels. Probably wouldn't have to mess with it for a very long time afterwards. I don't know of anyone that would have a spare for your current set up. Don't forget the bearing buddies.
Creativity is just intelligence fooling around