.... A lot of westerns feature almost mythical protagonists: the best gunslinger in the West, etc. Now, I'm not running a Western RPG, I'm running D&D set in the Wild West of Eberron (not unlike I'm not running a horror RPG, I'm running D&D set in Ravenloft, two different things). I'm just looking for good adventures that captures the feel, not necessarily recreates the genre...
You had me until that last sentence - because it contradicts your message. If you want an adventure that captures the feel of a Western (either have a western theme or can be given a western coat of paint) you need to consider the iconic elements of a Western. There is no absolute universal rules - but there are elements that are very prevalent.
A small group versus a big group of bad guys. Whether they're infiltrating and destroying a fortress, protecting a town, or rescuring the kidnap victims from the coal mines.
Anti-anti-heroes. They look like the type of anti-hero that you should not trust, but in the end the only thing they do that is non-heroic is be rude and kill bad guys.
Mounts. You've got to have horses or their equivalents.
Desolate setting. While a desert is not required, the surrounding does have to be inhospitable like a tundra or swamp. The environment needs to feel a threat.
Ideology. There is usually something ideological about the conflict. The heroes represent freedom while the bad guys represent oppression. The hero protects the people because they have the home he wishes he could have, but does not think he deserves ... and the villains represent his self hate. %$@# like that. This all stems from Westerns being movies and movies being allegories. Or porn. They can also be porn. Those are usually pretty straight forward, except the Western ones which are more farce - but even those include alleorgies.
By level 5 to 9 many of these features become less meaningful in D&D. Hordes of monsters are obliterated by a fireball. Who needs a mount when you can fly and teleport around? A desolate setting is pretty meaningless when magic provides food, water, shelter and summons for entertainment.
This is why I think of Western motifs as low level fare. The 'frontier town adventure' gives way to higher level challenges around 5th level where the PCs go from isolated challenges to regional challenges (followed by global challenges between levels 11 and 16 with interdimensional challenges for the highest levels).
Regardless, here are a few options:
Lost Mine of Phandelver. Replace forest with desert and boom. If you change the names and rewrite the maps most people will not pick up on the module being used - even if they've played it before.
Secret of Bone Hill. It is also very easy to convert to a Western feel for a low level party. In the late 1980s I converted it into the starting block of a mega dungeon and selected it because I was on a Western kick at the time and it had the right feel.
In the 1970s, TSR released Boot Hill as a Western Game. There were a few modules for it. They can be modified. Taming of Brimstone was in Dungeon or Dragon magazine (I do not recall which), but it was a good little romp.
Be George Lucas. Find a story you like, copy the architecture, and then put it in your setting. You could steal Firefly or another semi-western story ... but you can also take Saving Private Ryan or Dragonball Z and convert those stories into a western as well.
