The spindle will be part of whatever control switch it thermostat it is attached to, so you'll need to replace the whole switch.
There's plenty of places offering spares for cookers. All you need is the proper make and model and a good idea of which control you need.
Check out YouTube for many how to videos.
Remember to take paper notes of which screws and wires go where. Don't rely on memory.
Make sure the thing is unplugged or fully isolated before starting work.
If it's a gas cooker, you might like to ask a Gas Safe certified engineer to look at it.
If it's just the knob ,a replacement should normally be a push fit.