where can i buy table mirrors to decorate my wedding tables?

It depends on the size of mirror you are looking for.  If you are talking about mirrors that are around 6" square or smaller (these are easier to find), then these can be found in all the major stores like Wal-Mart, Target, etc for only a couple of dollars per mirror.  You would want to look near the craft section or the home decor sections.  Also, a great place to find these would be a craft store like Hobby Lobby.  They often have sales on these types of items, and you might be able to find a coupon for them in your local newspaper.  Saving money really helps when you're talking about a wedding budget!  Ordering them online from places like http://www.Amazon.com is also an option, but in my opinion, since it's for your wedding, I think you will actually want to see them in person before purchasing.

The first centerpiece mirror I like is the Octogonal Beveled Mirror.  This would make your table look elegant and it is easy to pick up and move.  It is easily stored too.  It measures 14x14x1/4 inches.

The second centerpiece mirror I like is the circle.  It is made of acrylic so it will not break as easy.  It is easy to travel with it and store it after your wedding is done.  This measures 18x18x1/8 inches.

The third one I liked is the Heart-Shaped Mirror.  This 6 inch mirror is perfect for weddings. 
There are 3 non-slip disks on the bottom of the hearts.  They do ship these in a bundle of 12.

I have also included some square mirrors to.  I hope these will help you in your search.  Congratulations on your big day.  Thank you for the question.