From my own experience, I don't think that subs are rated like "....2 ohm stable..." or anything like that. It is more important to make sure that the amp is stable to what ever resistance you want to run you subs.
For instance I had three Crunch Pros 4ohm single vc. I ran the off my punch 200ix @ 1.33ohms for over 3 years with out ever having a problem.
The key thing to look for in your subs, if you plan on cutting down the ohms of them, is to have a low THD (Total Harmonic Distortion) 'cause every time you drop the ohms you increse the distortion but also increse the out put power.
In your case I would get two 4ohm subs, run them parallel to get a 2 ohm load on your amp.
What kind of amp, how much power/channels are we talkin?