Replaced my ignition switch but lost the key to that replacement ignition switch?

I just replaced the ignition switch in my 2003 honda accord from another used 03 accord. The replacement switch obviously came with it's key.

However my mechanic apparently lost the key to the replacement switch and now I'm in a rut. Not sure what to do. What are my options?

That's what locksmiths live for.

That is the mechanic's problem; not yours. If he is a true shop mechanic he would not have lost the key. It would be on a hook or fastened to the bill to you. Or still in the ignition. The stuff stays together

So; want to try another story line?

mechanic 40+years

Darwin's "Survival of the fittest" includes having problem-solving ability. This one isn't rocket science.

Tech lost, tech replace

You did not replace the ignition switch. You replaced the key barrel. Get the mechanic to pay for a new key barrel.

The smart thing for the mechanic to do (if he was the one that lost the key) would be to take the new lock out and get the number off of it. Then call a lock smith to match the code. A real lock smith has a way to do that. Of course the barrel can be rekeyed too, meaning having the mechanism worked on so it takes a different key.
That way the mechanic pays for the mistake and not you!
If the mechanic bulks at this, sounds like small claima courts time (after you get the car out of the shop, not before).

You mechanic owes you a new key and whatever it take to get it. He also owes you a loaner car until yours can be used. If you can get the registration slip from the donor car and take that to the dealer, they can punch a new key using the VIN. If it's a chip key, it needs to be recoded to the car. Better get two keys this time.

If the mechanic lost the key, they own you a functional key, AND use of a vehicle until they fix THEIR problem.
If the ignition cylinder came from another 15 year old car, they CAN'T look up the cut depths because those records are not kept that long. A decent locksmith can cut a key to fit any lock cylinder in their possession. That isn't YOUR problem. The mechanic that lost the key is responsible to get it done.

Tell the mechanic you need the vehicle back tomorrow afternoon and that he should call when it's ready. Tell them you lost the key and it it's your responsibility to cover your mistakes.
If they try messing with you call the BBB and an attorney.

Tell them to call you when they find it. They need to find it and not be bothering you about it.