Have you looked into Diopter filters (also called Close-Up Lenses), rather than an extension tube? They can achieve the same effect, however they do throw another lens into the mix, so the quality of the diopter will likely play a big role in your ultimate image quality. Diopters come in a variety of strengths, from +1, +2, +3 or +4, and +10 in common cheaper sets, to single values, such as +10, in single-diopter high-quality lenses. Diopters can be stacked to achieve a greater effect, so a set of +1, +2, and +4 can get you up to +7 power total. I believe the value is the optical focusing power, although if that is always the case with all brands couldn't say. Regardless, they all have the same effect of allowing you to focus closer to your subject. Like any device that allows closer focusing, you DOF will become razor-thin if you stack on enough magnification.
You don't necessarily need IS for macro, and there is some question debate about how valuable IS really is for macro work. When working with such short DOF, you often need to stop down pretty far, and the best way to get a decent exposure and freeze motion is with a ring flash (rather than IS.)