A suitable (i.e. one who has the differences qualification themselves) Class Rating Instructor can do the differences training. It's worth noting that you don't have to have this done within a formal training organisation, you just need the instructor and access to a suitable aircraft.
For EASA, the "Acceptable Means of Compliance" & Guidance Material) at FCL.700 says that differences for a SEP rating are:
Variable pitch propellers;
Retractable undercarriage;
Turbo or super charged engines;
Cabin pressurisation;
Tailwheel;
EFIS;
SLPC.