Thanks for your replies....since I posted yesterday I have seen a few web sites that sell the parts but a bit steep priced for my needs.
I found a video of a guy who used a standard bike tyre and inner tube on a wheel with wood hub spokes and felloes, seems more doable, even if 28 or 29 inch is still a bit small. I want a single but very strong wheel to build a Chinese style wheel barrow where the wheel comes up through the deck.
Why do I want to make my own wheel? For pretty much the same reason folks make their own knives, shoes, bows and arrows, flint tools, clothes, tools etc....because I like making stuff and wanted to have a go at building something a bit more challenging (and precisely accurate) than another wood spoon! Theres plenty of compound angled mortices and tennons in one of those barrows....