Most electronic stores will carry flat screen tv mounts as part of a package deal, including installation. If you wish to install yourself, these can be purchased separately.
Online retailers like Best Buy are good places to look for cheap lcd tvs. Their ads are sales are always changing so this will require come time to keep updated.
Amazon does sell flat screen TV mounts but it would depend on what manufacturer of television you had brought from that would determine whether you would be able to get one that you require.
You can find them anywhere a mountable television is sold. Best Buy, Circuit City, even Wal-mart are all acceptable locations to find and size to your particular size of TV.
To wall mount a flat screen tv, you will need a power drill, measuring tape, a level, a pencil, and possibly a stud finder and a screwdriver, and you should really have a wall mount kit as well. A...