Well, it depends on who makes your grinder. If you've got a Metabo with the quick release, you will need to get the Metabo nut to retain that feature.
My 4 1/2" grinder has a 5/8-11 threaded arbor.
Going to my source of things I can't find elsewhere:
MSC has a 5/8-11 arbor nut from a Milwaukee grinder for $3.67
http://www1.mscdirect.com/CGI/NNSRIT?PMPXNO=2289131&PMT4NO=41304499Not bad, but the backing piece may be more of a problem.
They've also got the hub adaptor flange from a 7" Milwaukee for under $8, but here's the catch. That part needs to match up with the flats ground onto your arbor, so it cannot turn freely. I've got no idea if that particular flange will fit your grinder. Its possible (even likely), but I don't know.
For under $20, you could take the parts off a brand new HF grinder. I wouldn't recommend it for actual grinding though. :-)
I've actually got one hidden away for cutting tile and concrete, when I don't want that dusty crap going through my Metabo, but the awful racket its gears make just makes me sick.
A quick web search turned up something called the Jacobs Powernut
I don't see anyone selling this, but it appears to be a toolless nut, similar to the Metabo version. Still need a backer though, and over $20.
BTW, does anybody here have just one grinder?
Maybe you've just found the one to keep a wire wheel spun onto. :)