Not sure about Trailer tires, but a friend of mine started Tirebuyer.com. I got tires for my old Denali from them & installation and balance is included with the purchase.
Order them online, they ship them (for free) to a local shop (Seto's Here) and when they come in you get a call to go have them installed.
I have a 20' Hydra Sports on a single axle trailer....too heavy for the trailer I know. I purchase "U-Haul" trailer tires from Colony Tire. They are the only tires that will hold up for anytime. Other suggestion?