The Harmony 655 is still available on ebay, but if you buy a new one, it's gonna be expensive. However, it will have a warranty. Otherwise, you could buy a Harmony Hub, which can control all of your equipment by loading the app onto your tablet. You can use your computer to set it up- I prefer this method to using the phone or tablet, but it's not hard to use the tablet. I would recommend watching YouTube videos to see how it would be set up. Also, Harmony has info on its site, under 'Community'.
Was your RTi a T3?
Which "old tablet" do you have?