You can make your own masa via the process described by bozon, or you can buy preground masa at the store. If you live in an area without many latinoamericanos (such as, say, England) then you may be S.O.L.
to make it from corn
I have seen two types of masa available in stores: dry and wet. The wet is nice if you're about to do a lot of cooking, like if you were going to make 30 or 40 tamales
at a time. For cooking small amounts of food (pupusas
for 2, a couple tamales, etc.) the dry mix is easier, as it takes up less storage space and it keeps longer. The one thing to be careful of with the dry mix is that you need to pay attention to how much water you add. But, since you'll have your hands on it and in it while you are preparing whatever, it's not hard to keep track of the moisture.