Give this site a try. I have a Zinger..new last year, and the Duro tires on it are crap, sidewalls are cracked way beyond what I would even think of using. I shopped around, and I got a really good deal from this guy:
I bought Goodyear Marathons, and upped my size from ST205/75R14s to ST215/75R14s which gave me a higher load rating. He also claims the TowMax are good tires.
I bought 4 new tires mounted on 4 new rims for $102 each, plus shipping. The got to me in one week. He had quoted $87 each for the TowMax, plus shipping. I figure it will be easier to change the tires one at a time, so I bought them with the rims...plus his price beat all of my local tire outlets quotes for the tires alone!
I can't say enough about how well I was treated.
Like mentioned....passenger tires are not suited at all for trailers, you must have ST (or LT at a higher price) and proper load range.
Good Luck! Dave