... For comfort I find flanges < foam < Comply < custom moulds.
I haven't gone as far as custom moulds - yet; but then I like some of the things IEMs do, not others, and fundamentally rarely can stand more than 60-90mins or so of wearing.
Comply foam - my favourite, so far, agreed.
There are also quite-spendy aftermarket silicone parts that seem to get wide recommendation; like SpinFit. I've been through two or three of the latter in various sizes, picked based on knowing what fits my ears comfortably and they are AllRong / no better than what comes for free - and not cheap. Don't bother.
On IEMs themselves: I have tried a few. The ZS10Pros I've had c 3yrs - good enough, and cheap enough, even if the native response is a V-shaped curve is hilarious. Great on public transport for low-level listening and isolation; a mid/treble that will never annoy - but a thing I've noticed with such Balanced Armature devices - given the ridiculous sensitivity - they simply do not deliver dynamic range well; a really odd thing I do not understand. And it tracks even when I have them plugged into my diy class-A Can-driving beast-amp that could cook them with c. 8w still within class A
IEMs for me then
requires dynamic drivers. I have said before elsewhere on pfm I really like the Moondrop Aria. A single 10mm dynamic driver that is clean and well-balanced when given a good signal. A slightly-elevated bass level, but that is balanced by delivery of bass timbre better than many headphones of any kind at all. And - superb for IEMs at scaling across the range of music dynamics. The Arias represent as much as I will likely spend on such small/drop-able/fragile/ not entirely-sure they
really suit me, things.
Oh -then again I've long-owned a pair of the singular Stax in-ear Electrostatics; and after modding the driver box - they cream the rest: except, the in-ear fitting is _really_ odd/oval, so if you cant get them to seal - no bass! And the nominal idea of 600vDC bias pushed well inside the ear canal/ across the brain, might take some trust
ETA: July update below - have changed my mind, on SpinFit.
I stand by my comments on needing dynamic drivers; and the Arias remain a fave.