Indeed, Izirion's Enchiridion of the West Marches is really great.
Out of curiosity, has anyone ever released a "west-marches-focused scenario"? I wanted to purchase something, with the setting, the history, and the places/creatures/items to discover.
I imagine something like The Dark of Hot Springs Island, but more oriented towards a west-marches-style play.
Some smaller sandbox adventures/campaigns are good for this, because you can string a few together. Black Apple Braugh, Tomb Raiders of the Crystal Frontier, Beyond the Borderlands #s 1-3.
Bigger works can do this, but IME they may require considerably more work because you have to understand how all the pieces fit, cut the stuff you don't like (and all of the internal connections to that stuff), and then rebuild your material based on what the players actually show interest in, and where they start (even if just in the form of sticky notes or a one-page index/flowchart). Hot Springs, Oz, Neverland, Yggsburgh Vol 1, (the recently re-mastered) Dark Tower are all great for this.
Isle of Dread and Keep on the Borderlands, especially in their OAR remasters, sit in-between these two sizes, and thus are very good. You just have to supply a megadungeon and a few more points of interest.
The problem I find is that there are the good examples I mention above, but even those need a lot of work. And there are a million bad ones besides. If you want premade content, your almost always better off just finding or creating a big dungeon, and plopping down a bunch of much smaller, very interesting adventure sites, and then finding a broad-strokes way to connect them all of your own devising. Or go completely meta and just plop down in front of the players 2-3 short adventure modules you're already familiar with and ask them which direction they head for. (The non-meta version is give them rumors pointing to each module, and see which one they bite.)