If you do a search on "RAR Archive" on versiontracker (or the like) you should come up with several options, one of which will probably do what you want (although generally the point of RAR archives is to get them into smaller chunks--I'm guessing that 500MB has something to do with the capacity of a CDR, in which case couldn't you just send a group of smaller .rar files?)
I like MacPAR Deluxe, but although it will create PAR parity files, I don't know if it can also make RAR archives.
Alternately, you could always make smaller RAR files and then zip them into 500MB chunks...