I used to get them surplus from Micro-loco-motion but the owner sadly passed last year. These days I work with Jay Criswell at Right-O'-Way who stocks some suitable Maxon and Faulhaber coreless motors and gearheads for his drive upgrades that are great for O scale. Otherwise they can be found on ebay once you home in on the size, power and current draw parameters you need.
Some of the Maxon Remax RE25 series work well for O scale. Also Maxon RE24 with a 22GP gearhead can work well. Jay has several Faulhaber options also.
Pete