Fresh barrels are a bit of a problem for brewers, because the oak flavors can be so pronounced. Wine makers will use them, and often blend them with wine aged in older barrels to balance that out. After a few years, winemakers will often get rid of their barrels because the are now "neutral". Those are the barrels that lambic breweries and other sour brewers like Russian River, look for to replace their older barrels. These barrels are perfect for creating the hodgepodge of yeast and bacteria that make those beers, as the bugs will take up residence and live in the wood, and already started too when they held wine.
Most American breweries use bourbon barrels. There are many reasons for this, but most likely it is because they are cheap. Due to the laws regarding manufacturing of bourbon, the spirit has to be aged in new oak barrels. The oak gives bourbon that characteristic vanilla flavor. Once the barrel is empty, they are usually sold off. The other advantage to spirit barrels is that the high alcohol content in essence pickles the wood, keeping the bugs down to a minimum. Wine barrels, on the other hand, are crawling with brett and all those other beasties.
Old Bourbon barrels are purchased by breweries and other distillers. I know Allagash uses Jim Beam barrels. Scottish distilleries will use American bourbon barrels quite extensively, because they are usually aging their spirits for much longer then bourbon distillers, and thus want much of the oak taste already out of their barrels.
Also, keep in mind, most people don't actually primary in the barrels. The exception to that is lambics, but they tend to break all the rules. Most of the barrels are used for secondary. Once they get the oaky flavor they want, they will rack off, wash it, and refill it, until there is no more oak, or the barrel starts to go sour.
I think learning cooperage skills would be really cool, especially learning to disassemble and clean the barrels thoroughly, and then put them back together. You can not really sanitize a barrel, but you can strip it down to clean out the beer/wine stone and refresh the oak. However, I think you would find a demand for new oak barrels more in the spirit and wine world then the beer world. Good luck!
