There are a couple of posts here with schematics and links to purchase o-rings individually. If you do that, for $40, you can buy a lifetime of o-rings for your FX Impact/Crown. But, if you are lazy like me, I spent $40 and ordered from FX-USA. They will ask you for your rifle's serial number(different regulators, etc...), then send you an exact complete set of o-rings for your rifle/regulator combination.
The nice thing about buying from FX-USA? The o-rings are individually packed and labelled from FX-USA, so it jives with Ernest's videos and FX's schematics. The FX labeling then allows you to re-order from places like oringsandmore.com and replace the ones you used by going with the FX-USA labeling. So, as long as you keep the FX o-ring set packages and labeling properly for future use, you only need to order from FX-USA once...