I had experience with selling plasma and blood many years ago.
I'm sure that some things may have "progressed" a bit, but I suspect most aspects are pretty much the same...
Used to be, you could sell blood OR plasma.
When you sold your blood, the drew it out of you and kept the whole blood (and paid you - usually the "pay" was different for whole blood and plasma)
When you are selling plasma, they drew out your blood, separated out the blood cells (solids, if you will) from the liquid (plasma), then mixed your own blood cells with a saline or glucose type liquid, to replace the plasma that they kept, then put your "reconstituted" blood cells back in you. (and paid you)
Back when I saw it done, the blood was separated using a centrifugal extractor of sorts.... (spun the bag of blood in circles real fast till the weight of the blood cells made them go to the bottom of the bag, leaving the plasma "floating" on top.)
You got the snack - usually cookies and crackers & juice, which I assume was to help boost your blood sugar, after "losing" blood...
Some places even had "double" set-ups, where two people laid side-by-side, so someone could literally hold your hand, if you so desired...
Used to be, also, that you had to go one time and register & let them take blood to test - then, assuming your blood was ok to use, they would issue you a photo ID for the facility. After that, you just showed your card and they took you in and bought our blood.
What with modern technology, this may not be the case, these days, but, if you have a long trip or are counting on money that day, you might call ahead and ask if you have to wait to get "certified", or have blood testing results before selling...
In addition to the health issues folks have already mentioned (hepatitis, etc) that can exclude you from selling or donating blood, you can be turned away if you are underweight or currently ill (have a cold/fever, etc).
No one will take blood from me now, as I am 5'7" and can't seem to get above 104 pounds - too puny by their standards, even tho I seem healthy and appear fit...
As for facilities, look for everything you'd want in a doctor's office - clean environment, professional behavior, apparently competent staff, and sterile, single-use needles, alcohol wipes, blood bags, etc.
Most bigger cities have at least one facility.
Try your phone book's yellow pages, and try searching on line (maybe something like "my state + sell plasma" to start...)
Check the classified ads in city newspapers, too - sometimes facilities actually "recruit" blood/plasma sellers there.
You may also be able to learn more by calling your local county health department, or speaking to your local hospital.
People who's job it is to make sure folks have enough blood in them can be quite willing to help you fill the need...
Good luck - notyour