I can only give you advice on the Canadian Air Ambulance System. Basically most Pilots work full-time for the Ambulance. They generally require a CPL/IFR but an ATPL is preferred, even for a Co-Jo.
One of the biggest employers of EMS Pilots in Canada is CHC Eastern, now a completely management owned subsidiary of CHC.
If your looking at the Canadian market, then I would suggest you contact Rob Blakely at CH Eastern, in Toronto.