I just picked up some schweet 10" pnumatic tires on sale from Harbor Freight for $4.99 each.
http://www.harborfreightusa.com/usa/...o?itemid=30900
10" is a little smaller than I'd have liked but the price was right. The wheels are rated to 300lbs. and they have ball bearings for smooth rolling. While not as fast or as light as the larger nylon wheels used on most trailers, these wheels will allow for insane amounts of weight to be hauled. They also cost about 1/4 of what the 16" wheels I put on my last trailer cost.
I am still playing with a couple ideas for what type of trailer to build but I have worked out how to make a super-duty axle. The wheel use a 5/8" diameter axle so I got some 5/8" bolts that were about 8" long and cut the heads off them with my chop saw. I'll weld the now-headless bolts into both ends of a piece of 3/4" square tubing with just enough bolt sticking out to secure the wheels with lock nuts. (I'll try to remember to get pics) This will serve as a super-beefcake axle for the trailer. Below is an aski art representation of what I'm planning:
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The slashes are threads, the underlines are the unthreaded sectons of the bolts, and the brackets are the square tubing. I'll use nylon lock nuts to hold the wheels onto the axles.
Depending on what I decide the trailer should be, I'll either bolt the axle to a plywood base or I'll weld a frame from more of the 3/4" square steel tubing.