First point is your manual, showing you that all Canon EX Speedlite flash devices are compatible. (Older flashes are also usable, with certain limitations.) Differences between the Canon flashes can be seen on Canon website: http://www.canon.com.sg/personal/products/dslrs/eos-accessories?languageCode=EN#type=speedlite
Third party flashes can be found here in Buy & Sell / Mass Sales. An introduction into flash and the terms you need to learn provides the 'EOS Flash Bible': http://www.photonotes.org/articles/eos-flash/ (Get yourself a large cup of coffee / tea / whatever you fancy and take your time reading)
Main question is: what do you want to do with the flash? Which features do you need?
If you use it as 'extended onboard flash' then ETTL support will make your life easier. Take note of the swivel capabilities, important for portrait orientation of the camera.
For offshoe work you can either use Canon's wireless system, where you need a 580 type as master and any 580/430 or lower as flash slaves (multiple slaves are possible). Line of sight is required for the optical trigger signal and broad daylight might make it difficult. But: you will still have ETTL support for flash exposure.
Many people prefer offshoe flashes in manual mode with full control over the flash. For this, any cheap flash with enough power (Guide Number) will do the job and you need flash triggers. Popular radio triggers are about $60 for a set of transmitter / receiver. Here, the camera only gives simple trigger signal ('flash now!') and you need to set the flash power in advance on the flash unit. You can combine multiple flashes also. Do take note of the topic 'high speed sync' (see links).