You sound confused.
Filters go on the front (or in expensive cases "in") the lens. The diameter of the front of lens tells you what size filter you'll need for that lens.
Flashes go on top of the camera (normally), or are slaved from the camera as remote devices.
Canon flash guns for your cam are the 430EX and the 580EX, the latter is much more powerful and expensive.
There are similar versions by Sigma and Metz