On the Pentax 6x7 cameras, you need to slide the tension plate on the inside of the back one way (it has direction arrows to indicate which way) for 120 film and the other way for 220 film. And, before closing the back, you need to turn the little slotted button on the right side of the body beneath the winding lever to either 10 for 120 or 21 for 220. Probably best to do that first before loading the film. If you don't, the camera will not count the frames correctly. If you have 220 film loaded and have that button set on 10, it will stop winding after 10 frames and not let you finish the roll. If you have 120 film loaded and the button is set on 21, it will keep winding on after you finish the roll which is not so bad because you've finished the roll of 10 exposed frames. So set the button and push the tension plate to the proper position to accommodate 120 or 220 film accordingly.
The film costs $3.50 to $10 depending on whether black & white or color and 120 or 220, plus tax or shipping. Local prices could be higher.