I have also had problems replacing starters. I have ordered from RS components and had them fail.
It would seem that because the starters have been rationalised there are now starters able to work with a huge range. But often the 24" tubes are used in pairs and only the old starters will work in that case.
Also the thickness of the tubes vary so again different starter required. Even with same thickness you can get 20W and 40W tubes.
So sorry no real answer to your question as just too many options.