Anyway back to the thread.
You can pick up a Transit one on eBay occasionally, and the odd NOS autojumble if you're happy ploughing through boxes. However there are a whole stack of Lucas ones of similar size and shape but you will have to chop the plug off and rewire.
This shouldn't be an issue for the OP as they are fitting one to a car that originally came without one, rather than replacing like for like. As Arwyn said, the issue will be fitting it in the space you have and then remembering to cancel it!
You can get buzzers to help - I have one which buzzers after about 8 clicks, useful when changing lanes etc. It also mutes when the brake is applied so stops you getting irritated. Fairly cheap from eBay - search for motorbike indicator warnings.
The other issue you will have is stopping the switch sliding around the steering column.
If you look carefully at a Lucas 119SA indicator switch, they have a little plastic 'pip' cast into the switch body
This fits into a cut out on the steering column and stops the switch moving around. The OP will need to replicate this somehow! The only options I can thing of are to drill into the outer steering column or maybe make up a pierced brass plate which can be soldered to the steering column.
If you try to over tighten the retaining screws to stop it rotating then sure as eggs is eggs, you will crack the switch body
Arwyn