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.
I would buy a ready made one. Unless you are an excellent handyman, this could be troublesome. It has to support a certain weight and be able to tilt etc.
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.