Tlon↗ is the company responsible for creating Urbit, Landscape, Groups and Talk. Tlon↗ offers Urbit hosting for free, which includes a planet of your choice.
Planet One↗, a Tirrel↗ service, offers Urbit hosting for $15/month. You can also obtain hosting through their native Desktop application, Scene↗.
Holium↗ offers hosting for $15/month. Holium is the only hosting provider that allows you to join with an existing Urbit ID and pier, i.e. a planet that you have already booted elsewhere. If you don't have a planet yet, you can get one through their onboarding flow.