northern tool 16"5 SPOKED PNEUMATIC WHEEL...cheap as i could find with 1/2" axle.
$15 each last time i checked...good wheels...although when storing the big trailer on its side, the wheel flexed overnight and warped...it's not too bad, just out of true...I now store the trailer upright.
/edit: and the stock bearings on these wheels suck--we replaced the entire hub assemblies with higher quality cartridge bearings and included an aluminium spacer in between the bearings on either side of the hub which serves to pull the races of the bearings together, similar to some skateboard hub assemblies, via threaded axle and nylocks, shaft collars, set screws, etc. it's really not too complicated. pm me for blueprints.
link:
http://www.northerntool.com/webapp/w...9574_200199574