Honestly, if you're looking to start in macro photography, don't waste money on a lens or expensive extension tubes. You can get tubes for $10-15 on amazon. Play with those, get the hang of it, and if you want to delve deeper into macro shots, then consider dropping the money on Kenko tubes (all these do that the cheap ones don't is AF confirm and allow aperture control for lenses that don't have an aperture ring) or a macro lens.
I bought the fotodiox tubes for like $14 shipped and well, they do the job. I just keep a notes file on my iPhone to record apertures of each shot (I'm using an SMC Takumar for macro shots so I can set aperture with the ring). For purposes like these, there's no point in spending 10x on kenko or other name brand stuff.
7D - XTi - Tamron 17-50mm f/2.8 - Canon 50mm f/1.8II - SMC Takumar 50mm f/1.4 - Canon 70-200mm f/2.8L - Lensbaby 2.0 - Canon 430EX-II - CBS flash trigger