Yoke O'Clock kits are currently sold out, and I'm not planning to re-stock them in the near future. Any secondhand 12" LP will work, and you can find clock mechanisms at some local hobby stores, and online from various retailers. Try Lee Valley in the US, Wood Parts in Canada, Selva in Europe and Clock Shop in Australia.
Look for a mechanism with a shaft at least 16 mm long and no more than 22 mm. Look for hands with minute hand in 4-5" length.