Mint does not always grow true from seed, especially the ones labelled 'peppermint'.

You need to know people who will share cuttings with you! Attend a few meetings of your local Gardener's Club or whatever is available where you are. I'm pretty sure any social group will have someone in it who has what you want. You could advertise in your local shop window or local paper.

Or you can actually pay for whole plants, little ones maybe, at your local nursery, who should be able to order in whatever you ask for if it's not on show.

Or you could pop into the Exchange Forum here and advertise what you want - maybe you have something to swap.

My spearmint was started from some I got from the grocery store produce section. One bunch was a couple of dollars, and I got six good cuttings from it. I sunk a container in the ground to keep the mint in bounds, planted my sprigs and then covered it all with the bottom of a clear plastic container that had holes poked in the top. Within a week, I had a little bed of rooted mint cuttings. -You'll have the best luck doing this in a partly shady, moist spot.

Doing this in small pots indoors, then transplanting outside will give you better control over the environment issue dealing with diseases and bugs

after trying to grow spearmint from seed a few times to no avail, I ended up going to the local farmers market and bought a small mint plant for $2.50. As it grew, I took some cuttings and replanted them (let cutting sit in a glass of water for 4-5 days until you see a root beginning to grow, then replant). Now I have four very healthy mint plants.

As a side note, I tried one more time to grow spearmint from seeds just because I love the challenge. They are finally growing. I don't know what they'll actually turn into, but I suppose we'll find out!