If you can't search by address, you should be able to search by the owner's name, until you find the property address, then the lender's info. I don't know if the lender will be able to provide the info to someone who is not the owner, so you're probably going to have to be very persuasive. I would explain the situation in full and not stop until I had an answer.
I'd also try to find out if there's a 2nd mortgage on the property. That information is not likely to show up on the tax assessor's web site. But the 1st mortgage holder may know.
If all else fails, consult a real estate attorney or real estate title company and have them search the deed.
I decided not to rent the house for fear of being evicted and having all of my money stolen since he also wanted a hefty security deposit with a 15k move in.