Do you know of a commercial vendor for the switch?
there are quite a few. This
for example is a fancier version of my DIY cut-and-solder thing, that fits in a floppy drive bay.
You can go deluxe with something like this
which is much more complex (and expensive).
The first is just a fancier power switch, the latter switches the SATA connection as well (so you have only one SATA cable from the mobo for the devices connected to it).
You can easily find more (and maybe cheaper) by googling. You may want to look for hot-swap bays as well, although that is a bit more annoying as you will have to pull out the HDD in the bay every time you want to turn it off.
If you want to DIY your own (it is significantly cheaper) this
is a image-only tutorial that will guide you.