Believe it or not, if it doesn't come with one, you can't purchase one separately.
The manufacturers do that to be ghay. You need to warm up the engine, move the gear selector through the gears with the engine running, then put it back in park, raise the car on a (level) lift while still running, and remove a check plug to see if it's overfull or not.
If fluid doesn't drip out, replace the plug, add 1/2 pint, and repeat the whole process untill fluid drips out. When it drips out, leave the plug out with the engine running until it stops dripping, then replace the plug and your done.
See... totally Ghay!