How do I add the App Store back to my iPad?

You can’t delete the App Store. If you could delete it, you could never download any apps again.

You may have restricted it. Take a look at this.

Use parental controls on your child's iPhone, iPad, and iPod touch - Apple Support

You may may have accidentally hidden it in a folder. Swipe from screen to screen to look for it. Do a Spotlight Search. Swipe down from the middle of a home screen and type App Store in the search field. When it shows up, you will see the folder it is in (if it’s in a folder ) next the App Store Name.

You can also Reset the home screen layout in Settings>General>Reset>Reset Home screen layout and that will return the app to the home screen. It will also return the home screen to out of the box condition ruing any folder structure you had on the home screen.

It can’t be deleted so it has to be there somewhere. Check other screens and folders. Check to see if you have it restricted in settings.