The best places to look are:
I would also find parks by doing a targeted mail campaign or visiting as many parks as possible and asking if they would be willing to sell.
I find that a good manager is worth their weight in gold. We are able to manage our portfolio from afar (2,000 miles away) with good managers in place. We are also currently putting in internet cameras as well so that we can monitor the facilities from a computer anywhere in the world!
For out of state mobile home parks I would try to find a list (MHparkstore.com, MHvillage.com, etc.) The challenge will be that the owner information will not be available. In order to identify the owner you will need to access the tax office for each MH park. You could always address your letter to the manager/owner in hopes that it falls in the right hands.