I recently had a shower knob break and can not find a replacement knob that fits. I've been to Lowe's and Home Depot and the knobs I buy won't fit. It's not the knob itself that doesn't fit. It's the pipe that grabs the knob is too big and won't insert. The house is a 1970s house and I need to know where I can find a knob that fits. Any help is appreciated.

If you can find the manufacture of the part you can contact them.
Maybe post this question in the plumbing section. Tom (speedball) may be able to direct you.

Bet the best thing would be for you to get a plumber in and install a new pipe. Probably would be hard to get something to fit that fitting now. Would be nice to get a plumbers opinion anyway. Unless you're one yourself.