I can't speak to the difference(s) between the Schilke and Willson 51D as I've never tried them, but in regards to your second question:
Medium/euro shank mouthpieces are a little harder to find used, though they do appear from time to time on places like eBay, the Buy/Sell subforum here, and on Dillon Music's website. You just have to keep an eye out and check every so often. I found the Denis Wick I use on my Willson 2900 through eBay.
For new, in general you'll be looking at Schilke (though the euro shank is a special order from them, and as such is usually a little more expensive new), Denis Wick (the 4AM or 6BM I believe, the "M" standing for medium/euro shank. Same sizing as the 4AL/4AY and 6BL/6BS as well as the Steven Mead SM series), Griego (they restarted production of the Brian Bowman mouthpieces previously produced for DEG) and Giddings has a few mouthpieces in medium shank as well. There are others out there of course, but the ones listed are generally the more common brands you'll find new/used.
You can usually find at least a few of these direct through the manufacturer's websites (specifically Griego and Giddings), and/or through dealers like WWBW/Musicians' Friend, Dillon Music, Baltimore Brass, and The HornGuys, to name a few.
Willson 2900 TA-1 Euphonium - Denis Wick 4AM
Yamaha YSL-643 Trombone - Bob Reeves BrassArk 5G "Gladstone"
Yamaha YSL-8440 Trombone - Denis Wick 5BS
VMI 3301S BBb Tuba - Schilke Helleberg
Past:
York Preference 3067 Euphonium - Denis Wick 4AL
Benge 165F Trombone - Benge Marcellus
Wessex BR140 Baritone - Denis Wick 6BS
F.E. Olds Special Trombone (ca. 1941)