Most systems will have the sub integrated into the system (usually the controls, amplifiers, and other circuitry for the satellites and subwoofer will all be on one board with a single input and controller). It is possible to extract just the subwoofer from such a system but often it is much more work than anyone would want to do. I took apart a 2.1 system that died to find a digital audio processor tied to a digital controller on the right speaker through a serial link, not the kind of stuff you really want to mess around with.
It may be possible to find a powered subwoofer, but usually you'll only find "big" ones (ones designed for home theater use, for use with 500W surround sound living room systems, not your average little bedroom PC sub, probably be way expensive and very overpowering with PC speakers).
Like I said, if you're good with electronics, you can probably extract just a working powered sub from a 2.1 set and use it with the subwoofer output of your PC (assuming your PC is designed to output 5.1 with a subwoofer connector, with an iMac you may need some other sort of sound card or external solution).
The easiest way would just to get a whole new set of 2.1 speakers if you really want a sub, otherwise it's gonna be a lot of hacking around with electronics or investing in home theater equipment for your PC, which probably doesn't have the right outputs for a dedicated subwoofer anyways.
Want to Add a Subwoofer to my 2.0 speakers. 
Linear Mode
