Proper macro lneses are rather expensive. Canon's EF50mm f2.5 macro is £220, and doesn't have USM, and the Sigma's f2.8 lens is around the same.
You have some lenses with macro function, such as the Sigma and Tamron 70-300's which are a little slow and soft but otherwise ok. And there the brilliantly fast Sigma 18-50mm f2.8 Macro. Those two are your options for buying lenses.
However you can do macro on the cheap there are a few solutions:
1) Macro diopters/close up lens, these screw onto the front of your lens. You will need a 58mm lens. For example this one: http://www.warehouseexpress.com/buy-b-w-58mm-nl-10-close-up-sh-filter/p1013547
2) Reversal lens. You can get an adapter that will reverse the lens so the front fits into the mount. The EF50mm f1.8 (or any 52mm thread lens) should fit into the EOS mount backwards. This one for example http://www.flickr.com/photos/ciao_chao/4014610259/
3) Magnifying glass. Just hold a magnifying glass over the front of the lens and you will be able to focus onto much nearer objects. Works like a close-up lens. http://www.flickr.com/photos/ciao_chao/3759485546/
There are lots of options with varying price (from practically free to very expensive), and with different quality.