What should I do if a surgeon asks for an instrument that isn't available?

Just be honest tell them it isn't available and ask if he would like to use a simular one.

First, you should know if the facility has it. If it does, tell him you’ll get it and go get it or have it brought to the room. If you truly don’t have it, just tell him that you don’t have it. If the surgeon is adamant, call for the next person in your chain if command. Hopefully he asked for the instrument when he booked the case, if not, you can only do what is available to you. Should serve as a teachable moment for all.

Inform the surgeon about the situation and ask if there is anything else that you might be able to get, instead of that specific instrument.
I would also reach out to any resource staff on the unit that may be able to offer another suggestion.