I think patience is the answer.

I always wanted a few machines as I knew that sometimes they would be out of action due to needing maintenance, and if you want to play pinball you still can assuming the others work.

I knew the one machine I really wanted, but it wasn't available, so I purchase a couple of interesting titles as the other machines. Then when the one I really wanted landed I purchased that as well.

However, it sounds like you already have a couple of machines, so I would hold out for the one(s) you really want now.