What you should probably first consider if you're going to buy some new furniture is its purpose. A chair can have a necessary function in your home but it can also have a sentimental place. The difference between the two of them will direct you to a certain type of antique chair.
As you know, chairs arrived in our homes after the 16th century and only became common, really designed and functionnal by the 20th century. Chairs, old of 120 years, might be a little unconfortable for people who would like to use them as a primary furnishing. Usually if you're lucky enough to get this type of chair, you surely won't use it as a common chair but more as an precious antique object.
However, if the main purpose of your acquisition is for your personal use, you will probably be going to whole different type of chair. More confortable and conform to our modern living models. The links below refers to article explaining the different types of antique chairs, where you can buy them from and how to consider a furniture antique chairs in your home.