If you have it already installed on an iOS device you can do a simple backup to computer. For example, if the app is on another device, back it up to your MBP, and then install it to your computer. Connect your iPad and install it. Or check your Purchases.
But wait. I tried this with the Instagram app. I have it on my iPad 2nd Gen. but I can't use the one for iPhone. Also, some of the Apps that are no longer available in the apps store will not work with the current OS 11.xxx.
(side note) I've been doing this for years, backing up to my MBP apps that need to be in my iCloud and my MBP. For example, I don't have my Yahoo app backed up to my computer because I'm sure that it will be updated and available in the Apps store. I thought that was a good plan, but the older apps will not work with the current iOS devices.