I fitted these indicators to one of our electric trikes:
Mini Round Circle indicators Orange Len x 4 pcs new | eBay
I used the 24V power supply from the bike battery. 24V bulbs and flasher relays are easy to obtain because lorries have 24V electrical systems.
These are mini indicators fitted with small bulbs of 10W. Physically bigger bulbs of higher wattage don't fit in the indicator housing. The brightness is OK on most days, but on very sunny days they are difficult to see. By contrast, regulation indicators on my motorcycles have 21W bulbs and can be seen even on the brightest days.
I intended to use LED bulbs to save power drain on the battery, but I couldn't find a flasher unit that would flash with such low current draw. The flasher units were said to be suitable for LED bulbs, but I think they mean bulbs made of several LED elements, whereas the only LED bulbs I could get that would fit the indicator housings were single-element.
The indicators don't come with a switch. You could use one intended for classic motorcycles, such as item number SW19239 at
Indicators for classic bikes
I was also fitting front & rear lighting and a horn, so I used this unit:
http://www.electricscooterparts.com/images4/SWT-30B.jpg
However, the wires on this are very thin and so unsuitable for high current draw.
The front indicators hang from the handlebars, the rear indicators are mounted horizontally from the seat back-rest (I know not every bike has one of these!), in both cases using P-clips.
On the road I normally follow along behind this trike, and I do notice that car drivers respond to the indicators in a normal way. Perhaps this is helped by the positioning of the indicators: right at the car-driver's eye level, and well separated from the front & rear lights and from each other.