The United States Postal Service (USPS) has a website. If the stamp is still active, you should be able to purchase it there. Otherwise, check auction websites like EBay. There are also websites dedicated to trading stamps.
Search on the internet using pilgram stamp and the year that you want. Go through the options that are given to you and select a few and start cross referencing the quality of stamp and the pricing including shipping and insurance options. You should be able to find it easily.
My local coin and stamp shop in Denver carries a huge variety. I gave them a call and they have a couple of types of the landing of the pilgrim stamps. Check your phone book for a local store and I bet they will be able to help you out.
I checked in E-bay and found a wide variety of Landing of the Pilgrims stamps. They have some up for auction for the 1920 stamp and the commemorative 1970 stamp. Some are postal-stamped and a few were not. You might want to shop around there and see if you can find the perfect one for your collection.