We built our own, using a variety of leftover lumber we had lying around. It’s 5’ high; the center trunk is a 4x4 on a plywood base (about 2’x2’), with two 1’x2’ “stories” attached on 2 sides, and a top perch centered on top of the 4x4. The two middle levels were a bit teetery, so we added a 2x4 leg to help support them, with “L” brackets like you’d use for a bookshelf.
The only stuff I had to actually purchase was one cheap 5’ carpet runner (to wrap the center post), a couple of cheap carpet mats (to cover the base and 3 levels), and a roll of sisal (which we wrapped around the extra 2x4 support).
Cat uses the thing All. The. Time. He lounges on the various levels depending on his mood, the time of day, where the sun’s hitting it, etc.; if he gets hmself in a state, he’s downright aggressive in his rip-roaring climbing displays. (I should add that it’s parked right in front of a window, so the sunlight and views are the best he’s going to have anywhere in the house.)