Flashes?

I'd check eBay, I got myself an old 550EX which does me fine.

Basically you can get any of the Canon EX flashes and they will work quite well with your camera. Older ones such as 220EX, 420EX, 550EX will work just fine in ETTL mode. Newer ones use ETTL II mode that is a little more precise (many of the Canon lenses and cameras can also communicate more exactly the distance to the subject, based upon the focus).

The higher the model number, the more powerful it is. Also, they have more features, allow more control over various factors, or work better in multi-flash setups.

The 220EX, for example, among other things is auto only, can't do bounce (by itself), can't be used as a slave or master, can't be fitted with an auxiliary battery pack. But it's tiny and packs a pretty good punch for such a small flash.

A 580EX II, on the other hand, is highly configurable (or can be used very automatically), is about twice as powerful, has a head that rotates and tilts for bounce, can act as a slave or a master in a multi-flash setup, can be fitted with an auxiliary battery pack to speed up recycling and make faster shooting possible, yada, yada. Of course, it's comparatively big and heavier.

The 420/430 series flashes are in between.

Many third party flashes (Metz, Vivitar, Yongnuo, Sunpak, Sigma, yada, yada) also can be quite good, might not offer all the same features, but can cost considerably less. They might not be as well made or durable as Canon... Though some of the more specialized third party flashes are even more powerful or offer other features that Canon doesn't. But those tend to be pretty pricey, too.

I'd just be a little careful about third party flashes, do some careful research if you are considering one of them... Many are fine, especially the better known brands... But there are also a lot of unknown brands out there, which may or may not be good or reliable.

If you buy a Canon EX series flash, you won't have much to worry about. It will work well with your camera, you can pretty much rest assured.

Today's ETTL controlled flashes mostly are dead simple to use.... But many or even most also have a ton of features you can learn to use if and when necessary. It's sort of a direct relationship between price and power/features.