Here are a few popular mouthpieces available online:
Denis Wick Euphonium Gold Mouthpiece Medium Shank 4AM
This mouthpiece is great for older Bessons or the Willson euphonium, which all use a medium-size shank. It gives a much more open sound to the Willson especially.
Schilke 51D Mouthpiece - Silver, Medium (Willson) Shank
This and the next listings are the very popular mouthpiece made popular by Brian Bowman. I find it a little smaller sound than I want, but it is still a very nice-sounding moutpiece.
Schilke 51D Large Shank Mouthpiece - Silver
Schikle 51D Large Shank Mouthpiece - Gold Plated
Denis Wick 3AL Gold Mouthpiece - Large Shank
The 3AL was previously used by Steven Mead before he came out with his own line. Many people still prefer the original for its more open sound.
Denis Wick 3AL Silver Bass Trombone Mouthpiece
Denis Wick 4-1/2AL Silver Heavy Top Bass Trombone Mouthpiece
The 4-1/2AL gives a little more focus than the 4AL. Dan Vinson (Coast Guard Band) used it for some time and really sounded great on it.
Denis Wick 4BL Silver Bass Trombone Mouthpiece
The 4BL offers a shallower cup than the 4AL, which is great if you want a little brighter sound or a more impressive high register.
Bach 5G Bass Trombone Mouthpiece
If you are not ready for the larger 4G or Wick 4AL, this is a fine moutpiece. It is an especially nice step-up mouthpiece for players who have been using a 6-1/2, which is pretty popular.
Bach 4G Bass Trombone Mouthpiece
Similar sounding to the Wick 4AL, but a little more focused. The rim is also a bit "flatter", which some people find gives them better articulation and endurance.
Bach 3G Bass Trombone Mouthpiece
Bach Mega Tone 5G Bass Trombone Mouthpiece
Bach Mega Tone 4G Bass Trombone Mouthpiece
Euphonium BERP
Bach 1892 Nylon Trombone Mouthpiece Pouch
Reunion Blues Leather Trombone Mouthpiece Pouch