As far as hides go you could make them yourself, if you're up for it. It can be as simple as hot gluing some rocks together or as complicated as molding/casting something custom, or making carved foam hides that get textured and painted to look natural.

As for artificial plants, I've had good luck finding good looking terrarium plants at craft stores in the floral section. Just make sure they are either plastic or silk and there's no glitter/fuzzies on them or wire poking out the ends, and make sure they get cleaned before adding them.

The other option I've used in terrariums and aquariums are rocks from the landscape department at Lowes or Home Depot. Just be aware that some of their bagged rocks come with some sort of waxy coating on them that I had to boil off before use, honestly I'd boil or bake any rocks you plan on using anyway. It's just good practice.

Lastly you can pick up stuff from out in nature if you have an area around you that won't have been treated by pesticides or herbicides. There are multiple ways to make sure that you don't bring bugs or other nasties into your terrariums from nature, just search either here or Youtube on how to make sticks/logs/leaves/rocks safe for a terrarium.

There are some people who have used live plants, but I'm not really a fan. That's not to say it can't or shouldn't be done, I just like my enclosures to not need mowing/trimming. Lol