The connection between the stethoscope and your ears is one of the most important connections on a stethoscope. Not all stethoscopes come with a comfortable set of earpieces or a set which correctly fits your ear canal. An excellent stethoscope can be ruined with a poor connection between the stethoscope and your ears.
I have found several manufacturers seem to sell stethoscopes with the hard resin eartips by default and I have personally found the acoustics to be degraded by this style of eartip. If you do have a good pair it isn’t uncommon to lose one occasionally or to build up enough grime inside that they need replacing.
Many manufacturers have replacements available, but one of my favorite brands of earpieces are those provided by Littmann. The soft rubber earpieces on this brand have always given me the most comfortable experience when using a stethoscope. I personally use the large size, and they do an excellent job of filling my ear canal. Due to the hollow space that the earpiece is built around the rubber compresses in a fashion that makes the fit very comfortable. If I had any smaller ears I would probably go for the smaller size.
Exchanging these are a breeze. A couple small twists and you can easily pull off the OEM tips and then reverse the process to slip on these new ones. Depending on what brand of stethoscope you use you may have to experiment a little to find out if these will work for your exact model. There a subtle differences between the tube diameters on some binaurals. If you have time you could try fitting a coworkers Littmann style tips on your stethoscope before you venture out to buy a pair.