4 actually, RBCs, platelets and plasma are the common donations, w/ WBCs generally being used for autologous use (though it largely depends on the cell type.)
I think it's like donating blood. Correct me if I'm wrong, but I think there are 3 parts to blood: red blood cells, white blood cells and plasma.
Unfortunately, donor pools are decreasing by the year. Many blood centers do what they can to get new donors in the doors, though (I believe) many of the 'for pay' places use the donations for research. If you're a college student, check with your university hospital as many centers coordinate with them for these types of projects.
You might want to get some help if you are doing this just to buy games. Yeah, you are helping people but I doubt you would be doing this for free. Also, I'm sure they would just take the plasma if you donated it.