As the cheapest method possible, I have had great success with ironing on standard ordinary printer paper. The thing is you must compensate for the cheapness with a lot of care and effort. A few rules
1. Roughen the plain PCB with coarse steel wool to give the toner something to 'grab' to
2. Make sure the PCB is immaculetly clean and dry. I use detergent under hot water, followed by window cleaner to remove all traces of finger grease etc.
3. Use a LOT of pressure on the hottest iron you can get. Get the board very hot, then press down on the edge of the iron with as much force as you can muster. Cover the entire board.
4. The most important thing is to realise the paper does not release the toner. You must soak the board in water, and RUB the paper off with your fingers. If you do this, the tnor will not lift. NEVER lift or peel the paper off (If anybody follows this method, then at some point they will be tempted to try lifting, they will only try that once...). Keep the PCB wet.
5. When you think you've finished, leave the board to dry, any remaining paper fibres become visible, and you can continue.
I can get 15 mil traces using this method. Remember, it's cheap, but a lot of effort, and a small amount of trial and error, but quite repeatable.