macOS, or more specifially since iTunes v12.7, no longer stores the IPA files.
There are a number of ways to find an app’s IPA, such as the many untrusted, dangerous sites with their signed IPA.
Be safe, read on.
1. Download Apple Configurator 2
This is Apple’s app for configuring multiple devices for schools and businesses.
Download it from Mac Store here.
2. Sign In
Go to the app > Account > Sign In using your personal iTunes account.
3. Update The Apps
Go to Actions > Update, and select the apps to update (or all). If you have hundreds of apps to update, this process could stuck here.
Another way is to go to Actions > Add > Apps > Select the app to install from your purchase history.
After the apps are downloaded, go to step 4 immediately.
4. Find in cache folder
The IPAs will be here:
Note that they are in a cache, which means it will be deleted anytime, so copy it to somewhere while it is there.