Do you know the power consumption of the bulbs used in watts?
Basically, you've got 2 options:
1) Buy an LED flasher; these will work as long as you're drawing less than 23 watts (total, per side).
Something like this would probably do.2) Buy a Car/Motorbike flasher; these will work if you're drawing a minimum current, but this can vary. (They're usually rated in Watts. Add up the wattage of one front bulb and one rear to see if they'd be adequate). You can get these from halfords, or swap one over from another vehicle. If it's not going to work it won't cause any harm, it just won't flash.
Also, the first will be more regular in flash rate, but might not work with the original indicators.
I have a set of "3rd party" indicators; they work with an LED flasher unit, but they don't draw a lot of current (they're not LEDs).
Whichever type you get, make sure it's a 2 pin one.
EDIT: Also, avoid the units which come with "Load resistors." These cause extra power draw and will use up a lot more power then your indicators need to.