Model builders often have need of miniature worm drives. Making one's own can be labor intensive. One alternative is to use the tuning machines for stringed instruments. I paid $6 for six of them...
Disassembling them is easy. Simply unscrew the screw that secures the gear to the shaft and they can be broken down into component parts.
On the ones I bought, the gear has fourteen teeth so the gear ratio is 14:1. Other tuning machines may have different ratios.