I lost a turquoise stone in my bracelet. Where can I get a replacement stone?

Take it to a jeweler. They'll probably be able to find you a replacement and set it for you.

You can find loose turquoise gemstone from jeweler and get it fixed on your bracelet. Or consider replacing it with diamond. Loose diamonds are available more easily.

Have a look at :
Jeff White :: Fine Gems, Custom-Cut in the USA
Diamond Engagement Rings | Diamond Jewelry | Loose diamonds
just gemstones

Finding a stone yourself isn't the best option. There's no guarantee you'll find one that will fit properly, and you'll still need to take it to a jeweler's to get set, unless you have experience doing so yourself.

A jeweler will be able to find a stone that will not only fit, but also match the other stones in the bracelet.

Depending on what size stone you are looking for you can look online for a replacement or you can take it to a local jeweler or even a local jewelers club. If you are going to look online I would suggest looking at http://www.jamminggems.com