They're easy to find for the 383 and 400 engines, but not sure about the 313/318. It might be cheaper to order one direct from the US though.
If no one else chips in with some pointers try a google search for "gear reduction starter" maybe with some words like chrysler and/or poly
The only problem is Chrysler made these things many years ago, and you really want a modern one.
These guys look like they may be able to help;
Race Car Starters, High Torque & Mini Starters, Gear Reduction Starters