I've just reupholstered mine this week; ordered the foam and fabric from Exmoor and due to manufacturing delays it all arrived a few hours after we had left on an outback trip- this seemed somehow fitting as we were headed for Burke and Wills country! (History buffs will get this)
I'm sure you could do just the foam and re-use the fabric if it is in good condition The foams are available, I annoyingly only ordered one backrest foam and did not realise until I opened the box to find two of everything else and only one of these. Going to have my daughter bring it with her after her UK holiday in Jan as the shipping quote for one backrest foam to Aus was eye-watering.
A point that might be worth noting- the Exmoor fabric kit provides cable-ties for use where the original has wire rings. Far easier to use cable ties to refit your fabric rather than wrestle the steel rings back on.

