I suspect most people just use the indexes that came with their Filofax organiser when they bought it. Even in daily use they last for years. I still have some of my original ones from the 1980's in my Winchester Personal Filofax.During my 'moving in' session this evening to my new Pocket Chameleon. I discovered I had a choice of two different sets of A-Z indexes. One was a two letter set the other three letter set. OK it doesn't make a big difference, 13 dividers compared to 9 for the three letter set. But I was 'moving in' to a Pocket Organiser with slightly smaller rings so I went for the three letter set to save a bit of 'thickness' to my complete fill.
If you are having a reassessment of your main Filofax you might want to take a look at what indexes and section dividers are currently available on the Filofax website to see if there might be something slightly better on offer.
I've also seen several posts in the past where people have customised their section dividers. You can buy blank dividers or use a label printing machine to relabel your existing ones. I've done this on my A5 Finsbury.