As the owner of a pair of obsolete Ultimate Ears SuperFi 5 Pro earphones it's no longer to get spare parts from Ultimate Ears, (now Logitech), and increasing hard to get the parts from other sources.
I regularly lose the earbuds, and have tried a number of others, most of which don't fit the earphone properly and fall off/get stuck in the ear. These earbuds where purchased with no expectations. The earbuds were a tight fit to the SuperFi's and needed the flanges to be rolled back to ensure a proper fit, once on they seemed very secure, and so far (4 days of regular use), have not fallen off or come off in my ears. In ear I've found them to be comfortable and they form a very good seal resulting in very little to no spillage of sound & more a less total elimination of back ground noise.
Not sure about the sizing as I would normally fit the medium size earbuds, but with these I have got the large ones, (partly as medium were out of stock), I'm glad I did as the large seem to be a better fit. Normally large are too big for my ears.
All in all very pleased with these as a replacement for the Ultimate Ears earbuds.