John Deere 4052R Cab,, Deere 855D UTV, Z920A Zero Turn Mower and assorted implements
First place is to try Kubota for OEM parts. Aftermarket can be risky.
As far as the head, any automotive or truck repair facility should be able to fix it. Parts prices (new valves and such) are pretty much OEM...hard to find that kind of thing.
Cheap fix? Different story...unless you can do the work yourself.