How do I get a key to unlock my sliding glass door?

You can have a locksmith come but that would probably cost more than a new lockset. Perhaps you can take the lockset out and take it to a locksmith. Have you contacted the real estate agent to see if they can ask the previous homeowner if there is a key for that door?

Sliding doors usually have a manufacturers code engraved somewhere on one of the corners or a plate fastened that shows the name and the code. They can be called and perhaps a key can be made.

find out manufacturer of door and look them up on line. See if they might have replacement key for door or an answer for you

You could check with a local locksmith to see what your choices are and the prices. They would be the ones who would know. If you know the company that made the door, they may also be able to help you at least with finding out who made the lockset.

Hi Monica! We've replaced the cylinders on lock sets when the keys have been lost or need to be replaced. Much cheaper and easier than replacing the whole thing. Or a set like this seems reasonable:

https://www.amazon.com/d/Door-Handle-Sets/Prime-Line-1033-Sliding-Diecast-Non-Handed/B000I1VAZ0/ref=sr_1_20?s=hi&ie=UTF8&qid=1541434040&sr=1-20&keywords=replacement+lock+cylinder+for+sliding+door

If you can remove the locking system a lock Smith shop can make you keys.

Here you go:

https://www.popularmechanics.com/home/outdoor-projects/how-to/a26307/sliding-door-lock/

https://www.hunker.com/12566843/how-to-install-a-key-lock-on-a-sliding-glass-door

We use our slider with a key. The locking mechanism isn't that expensive, definitely less than a trip to the house from a locksmith. However, if it had a key and it was lost, call the manufacturer. They can often help.