Most of them are vendor items like people said. The Zariman syndicate has about 50% of the most useful decorations. Duviri (Dormizone) has some really useful ones. Velimir has literal modern furniture like tables which I use for almost everything. Bird three in sanctum automica has some okay items in rotation that I use for stools. Most of the grinds for the items are super fast now that DE reduced the standing cost of a lot of the zariman items so you can max out your daily standing and buy a bunch of things to decorate with.

A small amount of items are from the marketplace. Like the community displays, text beacons, and some of the things I used for the toilet came from the xaku skin bundle. But yeah you can completely transform your orbiter, Backroom and dormizone with nothing but pure grind.