The Graflex holders are among the oldest you'll find, but you don't need to worry about getting only graflex holders. The cameras take standard 4x5 holders, they are plentiful though you will likely spend much more on the holders than you did the cameras.
Heck, film will cost more than the camera.
For starting out, I'd stick with standard double sided holders rather than a Grafmatic. They can be handy, but can also be finicky to operate.