How can I restore an app on my iPad if it isn't available in the Store anymore?
If you had your iPad in sync with your computer, the app should still be in the "Apps" section of iTunes, available for you to synchronize with your restored iPad.

Alternatively, on your computer, select "iTunes Store", and from the "Quick Links" section on the upper-right hand side, select "Purchased", and then "Apps" (and then "iPad").  Some removed apps are still available for re-downloaded from that area.