My Jeep hauler trailer has ST205/75 D15 on a single jeep trailer. Trailer with the Jeep on it weighs in around 5200 pounds.

Tractor Supply sells them on the rim for about $110 each (quick, cheap, and easy. 3 very good things).
Discount tire wants to sell me just the tires put on my old rims, Carlisle radials mounted and balanced for $115 each. (carlisles are good)
I have found Bias ply tire and wheel packages on the etherwebs like easternmarine.com for $80 to $110 each(shipping is another $65).

Can anyone please explain bias ply versus radials for trailer tires only? Whats the best choice.

Can anyone recommend any other places to buy good trailer tires?

get Maxxis Radial trailer tires.. i think tey are called M8008

they are the best I have seen.. and yes I have them on my trailer..