Some people choose to buy used ones, some people choose to buy brand new ones, and other people just buy used chairs from the school or from the store. It's really up to you to decide what you want and need. There is no right or wrong way to go about it, so just follow your gut feeling and you'll be fine.
One of the best places to buy used furniture is on the internet. The internet has everything, including used books, movies, and furniture. This means that you can get to browse at all different types of places and see exactly what you are looking for. The downside to the internet is that it's kind of hard to trust, because a lot of people may pretend to sell you something and then later find out that it wasn't what you wanted or it wasn't even the right color. Make sure that you go with reputable dealers though, and make sure that they have good feedback from the Better Business Bureau.
Another option to get used furniture is at an actual store. If you don't know where to start, your first place should probably be Craigslist. You can easily find a good set of used chairs there. If you do buy used furniture from the store, try to inspect it to make sure that there aren't any major defects or that it still works. If you're buying online, be sure to inspect it before paying for it.