[MENTION=6802212]lobo316[/MENTION]
http://www.dndclassics.com/ is a store that has PDF and sometimes other options, for modules and other things from earlier editions. None are built for 5th edition, but you don't need to spend too much time converting. Most of the 1st, 2nd stuff is just taking percentile and converting to d20. DCs tend to go down a bit, add some more HPs in, lower AC. Basically done.
I just noticed that the Elemental Evil Players Guide is on their as a free PDF download as well as some softcover versions. I'll probably be buying it.
An anecdote - I'm in a tiny tourist town in central Washington this weekend. It's German themed and done up in holiday lights. Nestled inside one of the larger buildings is a comic book store. That store had one shelf of their RPG rack dedicated to 5th edition, a mix of spine and full cover frontings for each official book. It also had a regular Wednesday night DnD drop in session.
It's a town of less than 2000 people. It subsists off of tourism. Yet, they host DnD sessions and sell DnD stuff. And I'm certain that didn't happen in the 70s, the 80s, the 90s or the aughts. People that grew up playing DnD can now fund their passion in ways that teenagers cannot. They can get business loans. Their hobbies are aren't just passion projects, but money makers.
That's why DnD is stronger than ever. There are even 3rd generations playing it as a college kid in the late 70s has kids that are in their late 20s now and their kids are old enough to play as well.