For the amber lens I used 16099 Amber lens from http://www.vehicle-wiring-products.eu/VWP-onlinestore/lighting/round.php I turned them down on the lathe to fit the rim.
The light units are ordinary 12v MR11 domestic halogen down lighters (they are 20 watt so flash rate is about right) which you can get for less than £1.
It is also necessary to bypass the Stop / flasher relay and run a new wire to the rear. Connect a new cable to both of the existing stop elements of the stop/tail lights; remove green and purple cable from terminal 5 on the brake / flasher relay under the bonnet and connect to the new cable. Connect the cable removed from the original stop elements of the stop/tail lights and connect to the new flashers. Provide earth connections to new lights.
You will need to fit other reflectors under the bumper.
Whilst I was at it I also carried out a halogen headlight conversion using headlights with built in side lights which meant I could replace the front side light bulb holders with a single 21w bulb holder fitted with an amber colour bulb inside the clear lens. Vehicle Wiring Products also do the headlights.