It depends on what you actually need,as replacement wheels and tyres are available on eBay. As toy car collectors often replace wheels.
Failing that an email to 1st corps miniatures would source some from the World War, one range at Simon will sell white metal wheels from the range ,but only by request as they're not listed separately.
As mentioned Phil is quite a generous. Chap.So its definitely worth dropping him a PM.
There is a thread on here(somewhere but that requires a grown up to write a link as I've gone blank.) where larger scale wheels were used to rescale vehicles to fit better with 28mm,and quite effectively if memory serves.