I made a set of beach wheels.
I ordered Otter dock wheels from Mill's Fleet Farm. They are 24" diameter, 9" wide with a 2" axle hole. $86 for 2 including shipping. SKU # is 006364657.
Used 4 aluminum 2" shaft clamps from McMaster Carr, $56 plus shipping, and an 8' length of 2" OD aluminum tube with .25" wall thickness from Onlinemetals.com for $107 including shipping.
I put foam pipe insulation over the piping and wrapped it with duct tape, and put a rope through the pipe, going through a tennis ball at each end of the pipe. I figure the total cost was about $250. Also got 3/4" plastic plugs from Home Depot, drilled the drain holes in the wheels to 3/4", and sealed the caps in with silicone sealant to prevent water intrusion into the wheels.
Here's a link to a couple photos on Flickr: http://www.flickr.com/photos/100085529@N02/sets/72157635017809482/
It's funny, since I bought most of the options and toys for my Get: Spinnaker, wings, trap lines, marine radio, toy box, Torqeedo motor, Cheeta motor mount...but it rankled to spend $500 for Cat Trax. Although my home made ones aren't pneumatic.