Get a computer. Use the same apple id on your computer as you ios device.
There is a "hidden" way of downloading old versions.
#1# Assuming that you previously downloaded the app from the app store. Run app store on your ipad. updates > purchased. When you down load from the purchased section, you will get the version of the app for your device.
#2# How to get a new app you never download, but need a back version. convoluted. Download app on itunes running on a computer or on an idevice that supports the app. Use the same apple id you use on your ipad. Now, go back to the ipad. Follow step #1#.