Where can I buy diamonds?

I'm looking to buy a square-shaped stone to replace one that got lost from a pair of custom cuff links. I don't know much about diamond buying but is there a place where I can go to, preferably somewhere in PA where jewelers are so I can get lots of choices and a good price? It has to fit the cuff link because it is an heirloom and has a lot of sentimental value. Thanks.