The biggest comercial unicycle is the Coker (36" rim).
but your question doesn't sound like you're after a unicycle?
If it is for a unicycle, you should think about it cos, even though there are a few 60"+ wheeled unis in existence, they're not good to ride as the tires are solid, whereas the 36" coker is a proper pneumatic (air filled) tyre, hence why it's such a popular wheel.
If you really do need a 60"+ solid tired wheel, then the best place to get it is indeed from a company that makes authentic penny farthing replicas.
having said that, PKs advice to contact Roger from unicycle.co.uk, is a good idea- Roger almost certainly doesn't stock what you're after, but he's got a lot of contacts and will probably be able to point you in the right direction.
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