Gearmotor Assortment
Pololu has announced metal gearmotors with ratios ranging from 25-488. Rated 6V, small look nice but pricey IMHO at $20 each. Another buck gets you a second motor shaft for a motor encoder (but no disk provided).
https://www.pololu.com/category/167/20d-mm-metal-gearmotors
https://www.pololu.com/category/167/20d-mm-metal-gearmotors