I put thousands of shots through my FX Royale over the course of a couple of years and it never needed anything. I would assume you could get the o-rings from any of the retailers. Airguns of Arizona or Precision Airguns.
You have replace o-rings after thousands and thousands shots. You can find o-rings in hydralic shop, in guns shops, google (O-ring + model),whit tools and pacience is laburious but not difficult.
I replace my breach o-rings after every 1000 shots or so.
As for the size and type of o-ring required, you can find that info by going to the FX website and going to their support section. They have diagrams with part numbers and specs shown for various rifles. Here is the link: http://www.fxairguns.com/support/
The .177 cal takes a 4.3mm x 1.5mm Buna Nitrile A70 The .22 cal takes a 5.28mm x 1.78mm Urethane A70 The .25 cal takes a 6.35mm x 1.27mm Buna Nitrile A70 The .30 cal takes a 7.5mm x 1.5mm Buna Nitrile A70
Most of the o-rings used by FX are hard to find in typical o-ring stores. For example, here in the United States, only MD Metrics carries the 6.35mm x 1.27mm o-ring. MD Metrics website is http://mdmetric.com/.
wont work for for everybody as I have heard stories of them deforming pellet skirts on the way through but I haven't noticed any accuracy issues and they do last longer