I've finally taken the iPod plunge and bought a pair of Denon 551 in-ear 'phones to go with my Touch. The problem I have is that I cannot quite get the 'phones in properly; or get them to stay there if I do!

Which is a shame as when it works, it sounds good (and I've kinda got used to having something in my ears

).
I've tried all three sizes, and even mixing them up, but...
So, I'm looking to replace the supplied silicon seals. Any recommendations as what to try, and where to get them? I would prefer somewhere that supplies a mixture (I see that some manufacturers provide foam and silicon seals in varying shapes; but Denon only provided the silicon semi-spheres like those that Sennheiser supply) as opposed to 10x of one type/size only.
My preference (I think) would be for something that was quite soft and squishy on/in my ears (i.e. comfortable and not painful) but did a decent job sealing and staying in place.
I am assuming that different makes of seal are indeed interchangeable with OEM supplies...