Adorama.com and keh.com both have great used sections. I shoot Nikon, so don't really know much about Canon lenses, but one way you can save a bunch of money is by going with an all-manual lens. I don't think autofocus is all that helpful for macrophotography, and any kind of image stability shouldn't be needed as you should be using a tripod.
For example, I shoot my macros with the legendary Nikon Micro 105mm f/4 mounted on my dSLR. I got it for around $200. This lens was manufactured and sold in the 1970s, and still takes great photos. You just have to shoot in all-manual.
I also use an old Nikon AIS 105mm f2.8 manual macro lens on my Nikon D60 and paid around $200 at ebay used. Those old lenses are excellent quality. No need for AF for macro photography. The only problem is that metering doesn't work and I have to use the LCD display and play with speed and aperture to find the correct exposure.