The only type of leather you should buy is leather that is untanned. Tanning uses urea (guess where that is from) to finish off the leather. It is unsafe for birds.
All of our leather toys use untanned leather. Two particularly great choices for leather toys are the simple Thick Leather Strands and Thin Leather Strands. You can make your own toys with them, add to existing toys, or just tie it in knots and see if your bird can untie it!
African Greys and Umbrella Cockatoos (all Cockatoos, actually) really enjoy leather toys - they seem to like the way it sounds and feels when they squeeze it in their beak.