One thing that has really taken off in the past few years is the absolute mountain of goodies for WotC adventures. Doesn't matter which adventure you care to look at, there's just SO MUCH STUFF!!! And really pretty stuff too.
This is such a good point. My friend who's running Dragon Heist has told me he had to run down quite a few things to run the adventure, and it was out there! And all legit and legal-like. For my own game, I had to track down maps for Castle Ravenloft that work with a VTT and they were there! Someone went to a LOT of trouble to make those maps and I will be forever grateful. (I believe they are still up for sale if you're going to run the game).