I'm not sure which wiring on the trailer that you are replacing. If it is the "trailer" lights (running lights, stop lights L&R, etc) then I would buy the 7 conductor wire by the foot. I can get it locally for about $1.35 a foot. If that is the wiring that you are redoing, make sure you get the heavy duty 7 conductor wire with #10 wire for the ground and hot wire and #12 for the brakes. The rest are #14 for the lights.
If it is internal wiring, then you need to find the appropriate size stranded wire which will probably be an internet purchase to save some money.