Shroomy
Adventurer
FURTHER EDIT: Is there a market for high level adventures anyway? Six years of being in the Adventure Market make me think that answer is, unfortunately, no. From a customer viewpoint AND from a game designer viewpoint... High level adventures were ALWAYS hard to get folks to write (and write well) for Dungeon. The most popular level was 7th level.
Isn't this something of a self-fulfilling prophecy? I'm fairly confident that higher-level play will never be as popular as lower-level play, simply because of the time investment involved with getting to those levels, but if the system effectively starts to break down, requiring a lot of work and patience to get it to work right (if at all), then its going to directly impact its popularity and thus sales. If there isn't much of a market, then in general, people aren't going to focus their efforts on writing for it, and if they're not writing for it and not running it, then there not going to be able to get the experience necessary to write a good high level adventure, which is especially important IMO since one of the primary reasons why it is so hard to write a good one is because of the mechanical breakdown and all the extra work that it entails.