OB1
Jedi Master
Dungeon of the Mad Mage contains adventures for levels up to 20. I borrowed a couple of those and converted the fluff to fit into my campaign pretty easily. Which to Jer's point right above, took place almost exclusively in the Outer Planes once the party hit Tier IV.
But ultimately, high level play in D&D requires a big commitment from the DM to make it fun. At Tier IV, the specific makeup of the party has a huge effect on what challenges them. What would be trivial to one 18th level party might be an impossible challenge for another, which is what makes creating published adventures at higher levels so tricky to produce.
What I can say, is that if you can make that commitment, it will be worth it, both for you and the players.
But ultimately, high level play in D&D requires a big commitment from the DM to make it fun. At Tier IV, the specific makeup of the party has a huge effect on what challenges them. What would be trivial to one 18th level party might be an impossible challenge for another, which is what makes creating published adventures at higher levels so tricky to produce.
What I can say, is that if you can make that commitment, it will be worth it, both for you and the players.