iOS users can actually purchase the app prior to getting an iPad. You must install iTunes on your computer and setup an Apple ID/account and then you will be able to purchase the app via your computer. Then, when you get your iPad you will be able to install it on the iPad. Just make sure you sign into that same Apple ID on your iPad that you originally purchased the app with! Once you have signed into your new iPad with the same Apple ID, the app will appear in the “purchased” tab in the App Store App with a download link.
For Android tablet users unfortunately Google Play will not let you purchase apps without a device as they require a compatibility check with your actual device prior to purchase.