The only tire in this size I have ever seen worth buying is the Galaxy brand. They are OEM spec on Econoline trailers and I have had several sets. I have actually wore bald 3 out of 4 of them, over about 4 sets now. They are more expensive, and harder to find, but well worth it. If you can't find them locally, Econoline used to sell them in their parts department.
They are the only trailer tires I have worn bald, and got very high mileage out of them, with many of the miles loaded heavy in the desert southwest, where tire life is poor to start with due to the heat.
If you convert it, I would recommend going all the way to 17.5" tires and wheels. They will outlast anything else, unless you just get a bad brand of tire.
On Edit:
I checked their site, and while they have tires here:
http://www.econolinetrailers.com/800-14-5-14-ply-trailer-tire/
They do not say if they are Galaxy or something else. I would call and check before buying.