First, what kind of camera and what kind of lenses do you have?
I did a little research on extension tubes and found a thread that may help you...
http://www.photography-on-the.net/fo...highlight=tube
From what I've read, and this is just my opinion, but I think having a macro lense for macro shots is the best way to go, that is if you can afford it. Extension tubes are cheap, and in photography you truly do get what you pay for. I would recommend researching this topic more in depth to see if this really is what you want to do... also be sure to watch online auction sites as you can usually get a pretty good deal if you know what you want and what it goes for, that's how I got my macro lense!
