"LED Bulb Load Resistor 6 Ohm / 50 Watt For Signal or Brake light with Dual Filament LED bulbs upgrade "
From reading the page it's a 6ohm 50w resistor.
So it draws 2 amps or so.
You can also a trailer hitch plug in 3rd light.
The electronic flashers are good, and aren't load dependent for timing of the flashing.
RS wont carry these aluminum cased power resistors. RS has changed from a tinker's store to a life style store.
You should be able buy the reisistors, online at Digikey or Newark or Allied, or mouser.
But they are expensive. If your buying just one from one of these electronic component vendoers, it will be $3 -$7 +shipping and wont have wire leads soldered on for you, nor come with the quick clamp/crimp connects.
http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=240739
http://www.mouser.com/index.cfm?handler=displayproduct&lstdispproductid=257358
I did a quick search, mouser is usually the cheapest , but doesn't have 6ohm, just 5 or 7. 5. Either should work.
I would go with the 7. 5ohm, only about . 4 of an amp less draw then the 6 ohm, but less power in the resistor and should last longer.