Re: I'd like to get started, but don't know where to start.
You could install DosBox on your present computer and use it to learn all the DOS commands you want. Another alternative is to install VirtualBox or VMWare Player and install a Linux distribution such as Ubuntu that will allow you to get your fill of the command line.
You could even open a command prompt on any computer running Windows and use/learn DOS commands or Windows Scripting Host or Windows Powershell.
As far as having an old box to tear apart, you could check eBay as suggested but the issue there would be shipping as it would probably cost more to ship the old computer than it's worth. You might try Craigslist to see if you can find something local that you could pick up. Another option is to look locally for computer recyclers that may have older systems that you could pick up cheaply. I checked Craigslist and found a local, 10 year old working computer with AMD Athlon CPU, running Win XP complete with monitor, keyboard and mouse for $50. Saw another working P4 computer by itself for $30.
Heck, if you lived close by, I would be happy to give you an old computer.