As Bill says, it is illegal to have flashing headlights on a public road.
Please don't use them on the street!
Making a circuit is fairly simple if all you want is a simple L-R-L-R pattern.
A 555 timer in astable mode will work, as would the classic 2 transistor multivibrator.
A 555 will not drive a large relay. Use the 555 to drive a darlington or MOSFET and drive the coil of the relay with that, at 12v. Don't forget a freewheel diode across the coil to keep the coil on the relay from blowing out the transistor.
Here is a alternating flasher circuit using 3 555s
alternating flasher circuit diagram - Note the freewheel diodes. They are driving small relays. I don't know the specs on yours, but I don't think a 555 can supply enough power to turn one on without help.
You can also just use 1 555,
Here is the classic 2 transistor astable multivibrator: **broken link removed**
Basicly, you can switch transistors insted of LEDs with any of these and use the transistors to switch your relays.