well, I'm one of those DMs that would love to purchase a good adventure, since, as other have said before me, I just don't have the time anymore to create my own campaign & adventures from the scratch...
I have purchased most of WotCs adventures (City of the Spider Queen, Hord of the Red Hand and a few of the short ones) but recently I was hard pressed to find an adventure fitting for my group.
It isn't much of a problem to come up with little adventures for groups until the 10th or 12th level, but from there on adventures seem to become ever more difficult & need much more time to develop.
I have to agree with those who say that there aren't many adventures out there.
If I have a look at f.e. RPGNow... they do list 30 entries for d20 fantasy adventures levels 14-20. If you start looking at the actual products this comes up:
1 adventure location level 8-16 (no plot, just the location)
1 adventure location level 12-20 (no plot, just location)
1 Bundle (adventure also listed seperately)
1 adventure level 8+
1 classic conversion level 14-16
1 adventure level 18-20
1 adventure level 7-9
1 adventure level 13-14
1 adventure in a adventure series without levels mentioned
1 adventure level 7-9
1 classic conversion level 14-15
1 adventure level 7-8
1 adventure level 9-10
1 classic conversion level 15
1 adventure level 10-12
1 adventure level 6-8
1 adventure level 9+
1 adventure level 10-12
1 adventure for mid-level
1 adventure for 7th level
1 setting
let's sum it up:
there are 2 adventure settings that might fit into the level range, but most problematic about this level is not only the location IMO, but also a reason why characters should not just skip everything around there and just teleport, air-walk or whatever into the center... I don't know about those...
there are 3 classic conversions that just barely skip the level range but if you are not so much into those classic conversions, skip them
this leaves 1 adventure that just barely makes it near the level range and 1 (just ONE) adventure that could fit (level range 18-20).
using googles caching system (I love it) I even found JoeGKushner's review of it on enworld, which wasn't all that bad...
so, besides finding a high-level adventure at all (most of the few I found are part of an adventure series) - remember, I found 1 on RPG-Now, you need to find a reliable review of it, since you can't just buy something on the net because of the cover... (at least I don't, having had some bad experiences in the past).
I even tried to find an epic adventure for the past week and found out that there is no one who has ever published one (besides WotC themselves inside their epic book, but this one is crap).
Therefore: there aren't that many adventures out there. There are maybe hundreds of low-level adventures but once you start looking above 8th level the list becomes very, very thin.
Next you have to find them - and (IMO) they should be finding you. Maybe get a WotC-page with new products about d20 publishers? Maybe even get WotC to rate them?
Most of the publishers or resellers don't even have a category-system for adventure-levels. Therefore, more often than not, you have to go through all of their products to find adventures and then every 10th adventure goes beyond 8th level... try and do this on two or three pages and you can watch the hours pass...
Once you found one, you need to find a review of it, since you don't want to flush your money down the toilet. This is getting pretty hard since ENWorld review library went crash.
Now, most of the adventures rely on a background, which more often than not, doesn't fit in your campaign at all. I do play FR and there just isn't much out there that would fit into FR (IMO).
I am aware about the problems publishers have on the net, but on the other hand, everyone is building low-level adventures, you have real trouble finding what you r looking for (even on ENWorld you can sort through the reviews by publisher but you can't by book-type, in case of adventures by level range etc etc). And there r very few reliable reviews out there...
yes, it would be nice to be able to use more of the WotC-Stuff in those adventures, but I do believe, that it wouldn't change much. The problem (and therefore the reason, why I rarely buy electronic products of 3rd party publishers) I am having is Information and Quality. As long as this doesn't change (and ENWorld is doing a better job about changing that with News and Reviews) significantly the market for adventures on the net will be too instable, unrelyable and too small for publishers to put much work into adventures... just my 2 cents