Morning Matt,
You can either make it in wood yourself and see if you can pick up Indespension units and wheels up from somewhere like Ebay, or you could knock one up from something like scaffolding tube if you have someone nearby who can weld.
If you do it in wood, then there's no labour cost as you can do it yourself, but I'd expect any welder to be able to weld one up in less than 2 hours, and if you do it in scaffolding tube then sometimes you find bits of it dumped so the material is cheap.
You'd need to work out how to get the harnesses to fit it in place though!