Sony DAV-C990 Stereo Speaker Options

Hello,

I was wondering if someone could help me with an audio question for the above stereo system. I have the complete surround speaker system, but, unfortunately, the four tower speakers are really too much for my system usage other than the sub and center channel speaker. What I am curious about is finding very small appropriate speakers that I can use in place of those provided, which are 3 ohm speakers. It seems to me that most stereo speakers are 8 ohms, so what I want to know is whether I need to use 3 ohm speakers for the system to work. Are these a common type of speaker to purchase? If so, where is a good place to look. Any help would be greatly appreciated. Thank you in advance!