Personally I would go to Robsport. Used them for years and they don't sell rubbish, or if they do they tell you (e.g. rubber carb mounts!!). Used them for years for loads of Standard Triumph Spares, never had an issue. The one they advertise is a "Lucas" one, of course we all know this is not the original Lucas, "Prince of Darkness" Lucas, but it is probably as good as you are going to get.
You don't say why you need a new one but before fitting just make sure the connections (the riveted ones in the switch itself) are secure and the switch action is good. Usually with light switches of this era it is the heat and or arcing generated at the switch contacts that cause an early demise. The dip function is the one that usually goes first as it is carrying the current for all four headlamps. In the 25 years I have had my 1985 Landy 110 - similar switch - it has "eaten" 2 of them. I have now fitted relays to the headlamps to take the heavy current away from the main light switch and the dip switch - did the same on my Stag and I am about to do the same to my Sprint, just as soon as I get the front suspension back together
Apologies if "teaching granny to suck eggs"