You can use the older books. While the setting has been updated with 5th edition, not all older material is invalidated. In fact, most of it is still usable. And you should consider the lack of material not a problem, but a challange. If there's nothing (or not much) background on a location, you can play there without the rule lawyers amongst your players going 'that's not correct', because you make the setting yourself. It's a bit more work, I grant you that, but it can be very rewarding.