If you want to save some money on the mount and can wait a few days for it to arrive, I highly recommend buying from monoprice.com. I have ordered many items from them, and they have the best prices on cables and mounts. Don't let the cheap price fool you, they are an excellent mount. I have purchased 2 different ones from them and they are really solid, and the instructions are very easy to follow. Our school district has bought all of their flat panel mounts from them and even the skeptic installer was very impressed by the quality and ease of installation.

Here is a link that might be useful: Monoprice

Please read down a few posts for a similar post with information regarding your question.

Several of the big box stores that offer installation found out the hard way of the problems with over the fireplace installations and don't allow their installers to perform it anymore. As an example, you can view the restrictions on the Costco website for TV installation services that they offer through a third party installer. I'm willing to bet that if Best Buy would do it, it would be a separately quoted, expensive installation, with several disclaimers.

Monoprice can be a good source of product at a very good price. I've found their wall mounts to be good quality but highly recommend purchasing higher grade mounting hardware, such as the lag bolts, locally. The heads on the bolts appear to snap off far too often while tightening.