I've read that consumer level downgrades are hard to come by, even if you have Win8 Pro. The downgrade has to be supported by the manufacturer, and I have the strong impression that most of them don't want to do that.
Another alternative is to simply buy Windows 7 and install it, as if the computer came without an OS. That supposes that you can get all the drivers
you need to run Windows 7 on the hardware. (Shouldn't be unreasonably difficult at the moment, although laptops and all-in-ones may pose special problems due to driver
customizations done by the manufacturers.)
I'd probably use a System Builder license, in violation of the license agreement. (I can only speculate as to what fraction of S. B. licenses sold by Newegg are used that way. 50%? 90%?) Still expensive, though; roughly $100US for Home Premium, $140 for Pro.
For my own purposes, I'd stay with Win8, and install one of the Start Menus restoration utilities as a crutch. And hope that Microsoft changes their minds soon about the level of configuration permitted to Win8 users.