Just about anything you can find in vsti that's OSX compatable can be found in AU and RTAS. Everything else is gonna be left behind in the switch to Mac anyway.
If they made it in AU(apple's plug format), they more than likely made it for every platform out there and included a RTAS version. Even compeditive DAW companies(Cakewalk, Image Line, Steinberg)have RTAS versions of their instruments last I checked.
You can find tons of synth strings between Arturia, IK Multimedia, and Native Instruments alone. You probably already own a few RTAS if you refer back to your install disks or go back to websites log in and look back at what formats tools you own can be downloaded in.