I think the answer is a"heavy-duty flasher relay". They provide a constant flash rate and are not sensitive to load changes. I've used them in the past and they work well. They are a form-fit-function replacement for the OEM relay. Not sure where you get them as they don't seem to available in this country upon a cursory check. I now you can get them int he states at any auto parts store.
Have a look here:
Grote Industries
Hybrid Mode
