You can obtain Tims in 1 of 2 ways:
Collect Tim Eggs from levels. Pay attention to your companions they will often bring you eggs you may have missed your self in the field. Find the egg(s) when you return to the tim island and run into them to hatch them and add them to your gruesome horde.
Breeding tim. Feed a tim 30 or 40 yellow crystals and it will become HUGE. Throw any color tim at the fatty you can pick up, and an egg will pop. Run into the egg and it'll hatch.
Build your nightmare army DO NOT USE ANYTHING BUT YELLOW CRYSTALS to feed them. You'll need as many as possible to help you get other cheevs done.