You can grow mint in a small pot outside, MyNameIsTerry, and then you can just pick what you need. It is very invasive, so it's best not to grow it in your garden, as it will out-compete almost everything else. Buy the common mint, rather than one of the speciality ones they sell in the garden centre.
Pick and then tear the leaves roughly, add boiling water and leave for about ten minutes to brew. It smells wonderful, and fresh mint is much stronger and more effective than commercial mint teas.
Oh, and I've just checked, you can grow mint indoors if you don't have a garden.