Be careful with an aftermarket one.

Not all of them may be manufactured or rated for actual rollover protection. I think the OEM structures specified particular steel ,  mounting hardware and probably had to be put together by certified welder.

There was this site that was manufacturing all sort of bars and canopies for many different brands but it appears to have ceased production and closed ? 

http://www.saginawcountytractor.com/

Stay safe and remember a ROPS can't protect you if your not held in place by a seatbelt in the protected zone.