Personally, I'd get one from one of the LED suppliers so you'll know that the flash rate won't be load dependent (unless ballasts, resistors, etc. are being used).
From SuperBright, I believe CF13GL-02 is the appropriate relay - note the orientation of the pins (B=battery, E=earth, L=load).
This was confirmed this evening when I replaced my 4-prong LUCAS 52010208 AMR 4873 12.8v flasher relay with the above 3-prong CF13GL-02 SuperBrightLeds unit. Took about 5 minutes after assuming the Lotus position with the top of my head pressing against the brake pedal, facing upwards. You literally just pull the old relay out and pop the new one in. It's the one farthest away - when looking downwards / towards the rear of the car. It actually took longer assuming (and unassuming) the position than it took to do the swap.
Anyway it works perfectly! Runs all the new RED LED's in the rear.
Here's what I ordered to get LED's in the rear (tail/turn/brake lights):
Product: BA15 24-LED bulb $ 7.49 (each)
Options: Red 1156-R24 Wide
Product: BA15 4Watt LED bulb $ 16.95 (each)
Options: Red 1157-R3X1W
Product: CF13GL-02 LED bulb Flasher Unit $ 12.95
It's VERY bright, even in direct sunlight... and they turn INSTANTLY... and now they flash properly. No resistors needed!
Total of $110 + shipping. Not too shabby!
I'll post pictures later.