I'm interested in making a joule thief and doing some testing with it. One design I came across has a common mode choke with 2 wires, and 4 ends. One wire was wound around a ring magnet 100x, and the other wire had 8 windings. Does anyone even sell something like this? The ring magnets I have are too small for that many windings.

The final use of this joule thief would be in a low voltage LED flashlight, so it would be using max 3.5vdc and about 30-40ma.

 

Pre-made common-mode cokes would normally have two identical windings with not many turns in each. You can buy toroidal cores in various sizes for a home-brew Joule thief transformer. You may also be able to salvage a suitable core (not ring magnet) from junk electronic equipment.