Trailer build questions

[TRUCKMAKER]

The best place I've found in the states for kits would be Tower Hobbies. If you plan on being in this hobby or any RC hobby for a while check out their club member ship its half price right now and you get free shipping option and deals all the time. I've shopped this stuff for a lot of years and haven't found any one cheaper on the big stuff on a regular basis, you might find a deal once in a while on ebay or amazon but usually not when you want it, but it never hurts to check.
As far as the aluminum Cossett and fhhhstix referred to, if you have a Menards near you they will have it. Ace hardware or any large hardware store now days has it.