Where can I buy a double door lock?

What you need is a "communicating double cylinder lock" keyed on both sides. They are a violation of fire codes in many places throughout the US and Canada, so they are hard to find in stores. See a locksmith.

Safety first.
Yes, I warned people, no one is listening.

My suggestion:
First, you will never ever leave her alone or unsupervised.
Second, you can alarm the doors instead.

So sorry you are going through this with your wife. It is good that you came on this forum, Warren, because people who have and are experiencing the same with a parent or spouse can walk you through each issue as it comes up. Stick around. imo.

On this Sunday past, a fire inside a home, in a remote hilly area, an elderly couple died because they had locked or even barricaded themselves inside their home for security. Their rescuers had said they couldn't break in with a stick. On the news, a video of the fire showed french doors with glass windowpanes. When the fire dept. came (which was right away in L.A. County) the couple were transported in critical condition and later died of smoke inhalation.

This sad circumstance touched me not only because the people died; and not only because the National forest was threatened by the house fire; and not only because it was down the block from where I was housesitting (I was there); but also because, when I smelled the smoke and saw the helicopter hovering overhead at the street level, it was then I realized there would be no possible timely evacuation for my elderly aunt and uncle with their pets in a nearby fire.
Maybe they will get out in case of fire.

Put a hook and eyehole lock up very high up so she can't reach it. But again think about safety and fire codes.

For others looking for this item I found one here:
homedepot/p/First-Watch-Security-Polished-Brass-Keyed-Knob-Mortise-Lockset-1129/202799848

I agree with Glad, I have posted elsewhere against the use of double keyed locks, but if you make an informed decision to use one then so be it.

Be careful of locking her in especially if she is ever home alone. This would be considered elder abuse and could get you a citation. If there were a fire how woyld either of you get out? A panic dash for the keys. "Now where did I put those darn keys!" And you both are lost in the fire.

Warren, I'm not sure what you are trying to do, usually you have your regular door nob or lever to open and close the door when you need to enter/exit and the deadbolt as a separate lock above that to secure the door. If your door doesn't have a place for a separate lock a locksmith can put one in for you, even on a metal door. I put one in my door when I moved in, the cost was very reasonable.

As I said, there are plenty of double keyed deadbolts in the local stores. But deadbolts don't have levers or knobs.

Double keyed deadbolts are not legal in some places as they are against fire safety codes, that could be why you can not find one in any local stores.

We just put in one of the door code dead bolts...about $35. Could you put one on each side....works great. You can call a locksmith and see what they have or ideas?