For good and bad Xplane has endless free contributions.
First of all, you can begin with the files here in Simviation. Taking into account that Xplane 10 was replaced some years ago, you will get files beginning searching from page 11 backward.
Depending of your preferences there are several sites that have good freeware for Xplane 10 and even previous versions. If you make a list I can help you, but remember there are endless files.
Go here and you'll find a scope of addons for Xplane: https://forums.x-plane.org/