The big 2 comercial vendors are Vision RT and C-RAD. There are open source librarys that have been used to create research based systems or if you are so inclined can be used for building your own 1 off system.
I heard there was one more potentially commercial system from a smaller company at the AAPM spring clinical meeting in Salt Lake back in 2016 but the name escapes me at the moment. Maybe someone else on this list was there and can remember. I believe the speaker was David Shepard from Sweedish up in Seattle.
Best of Luck
Mike