My dining room/great room area is sloped, with a single layer of cedar boards for the ceiling rather than a standard sheetrock ceiling with room above in an attic. There is no room above the ceiling to drill into and install an electrical box. The current lighting fixture is a chandelier that is physically mounted on the ceiling itself--which IS very stable and solid--but instead of being wired directly into the ceiling, there is a plug-in cord that goes from the fixture to a cedar beam a few feet away. This beam does have a wiring box, with a plug, connected through the wall, to a dimmer switch in the wall.

So, the two issues we must solve for a ceiling fan are:
1) sloped ceiling (not so hard to do probably)
2) very different wiring than usual

Does anyone know of a ceiling fan that can easily convert to a plug in? Or should I just wire it to a regular power cord myself?

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You can mount the fan directly to the wood support, and then run the wire to the nearby outlet box. All wiring should be done inside the fan's canopy (which counts as a metal box) and make sure you remove the dimmer if you wire the fan to the plug it controls.

You will want to make sure the fan is mounted to a joist or crossbeam in the ceiling, not just the cedar paneling.