Here is a handy (but limited) cross reference of various core materials from various manufacturers. The material type, such as 43 or 73, applies to types of cores other than toroids. The Fair-rite 2673002402 uses 73 material and makes a very good choice for a Joule Thief core. Another choice is the 2643002402, which uses 43 mix, but has lower permeability and takes more turns for the same inductance.