This is quite a good link - as it mentions a number of companies which produce retractable and detachable wheels for SIBs. You should be able to price the wheels from their own websites. Also some useful information about maximum load weights, etc.
Inflatable Launching Wheels: How To Launch A Small Boat Without A Trailer
Zodiac are probably one of the biggest boat manufacturers in the world (also own Bombard, Avon, etc.) Their stuff isn't cheap... with a set of wheels retailing for around £250. Look up Zodiac codes Z61110 or Z1290 for more information. Not a bad profit margin when you consider it's no more than a couple of aluminium shelf brackets and wheelbarrow tyres from B&Q!
I'm sure you're already aware of Sealeg RIBs, but if not then here you go: Sealegs Patented Amphibious Technology