The ratchet may be salvageable without using new parts. As long as the spring behind the ball is still good, you'll be ok. The ball may have rusted in the depressed position, and you may be able to free it. It will take time. If the spring is broken, then you're SOL (it can be fixed, but it's easier just to replace the head).
Try soaking just the ball area in vinegar, EvapoRust, CLR, WD40, or PB Blaster. Keep an eye on it, the chemicals can eat the spring. Every now and then, try working the ball with a pair of channellocks (or pliers) by gently squeezing and releasing. The idea is to get the rust converter/inhibitor into the detent behind the ball in order to free it up. then clean out as much of the rust/crud as possible.
Once free, rinse the detent area well. You will need to neutralize the chemicals so they stop working. Dry with a heat gun or hair drier. Then oil it well to prevent the rust from returning. Use the channellocks to work the oil into the hole. Lube or oil the ratchet gears as well. If the detent got wet, then the ratchet gears may have as well.
If all goes well, you will have a working ratchet again without spending any money.
Good luck.
PS - If the ratchet is a "quick release" different rules may apply but the principle is the same.
