The ones that girls flash their tits for.
Are they purchased, or given out by the organisers.
When did the beads first come into use.
Spring break where? The only tits-for-beads deals I’ve heard of are associated with Mardi Gras in New Orleans. Google “mardi gras beads flash”. The beads can be bought cheaply at many (if not most) stores in New Orleans.
When I’ve been in New Orleans, I’ve found lots of shops selling large ranges of strings of beads. So presumably people buy them there,
(1) to wear themselves as part of the celebration,
(2) to take home as souvenirs, and/or
(3) to throw at young women exposing their chests.
I’d guess that in other places where young people go for Spring Break there are similar shops.
We got them in a Party City three states over. You can get them at most dollar stores. They are used for more than Bourbon Street ya know.
From the late night TV ads for spring break videos, it appears as though beads are stock in trade for that sort of action. Not that I would watch those sorts of ads, instead of skipping past them on the DVR. 
Generally you get relatively cheaper ones tossed to you at parties or from floats (depending on where/when you are) for free. Enterprising individuals will buy better ones at dollar stores or Walmart type stores.
Well, OK, it’s not actually relevant, but it should be.
Buy them in bulk
Go to the Mardi Gras supplies.
This is one of my favorite places… and the showroom/warehouse happens to be right next to my work. Explore the site and have fun. This is where I get all my holloween candy.
I just did the math… you can buy 72 strands of beads per bag… as long as you buy at least 20 bags for $12 per bag. That comes out to just under $0.17 per strand. I figure you could cut out the middle man and just tell the co-eds you will give them twenty cents to show you their boobs and make out just as well.
I remember our school mardi gras fundraiser. When I saw them buying beads from the dollar shop, I thought it was going to be a completely different type of fundraiser.
In my experiece at Mardi Gras the way to get beads was to watch a couple parades and catch them there, and then head to the French Quarter where most of the debauchery goes on. But yes, there are plenty of stores selling beads, but in my group that was kind of looked as cheating.
Keep in mind that most women aren’t going to be flashing for the cheap strands of beads. Longer strands, strands of larger diameter beads, blinky beads, etc. are “worth” more and will attract more attention. Maybe it’s different on spring break where there is less history/tradition to the whole ritual, but dangling a strand of party store beads to try and get someone to flash at Mardi Gras is probably not going to work.
This journal article (link is just to abstract), Ritual Disrobement at Mardi Gras: Ceremonial Exchange and Moral Order might help answer the OP’s question of when the beads came into use.
if you just want a strand or two, all our local party stores have them for about a month before Mardi Gras.