THe correct deffinition of Macro is an image size of 1:1 focal length dioes not matter for that it is design. However with a longer focal length you can be further away from the subject and still get macro. The Sigma 105mm macro is an excellent choice.
Extention tubes are OK but they do loose a lot of light so you need a really wide aperture lens top start with.. or loads of light on the subject.
Another alternative are close up lenses screwed into the filter mount. Again need to use a small aperture for decent sharpness.
Finally you can get a reversing ring for your standard lens. This allows you to conect to the body the other wauy round and gives a macro effect. You will need a lens with apertures on it otherwise you will not be able to get the right exposure.
eBay is a great source for all the above.