These quizzes usually come from all sorts of different websites, however there are lots of them here: http://www.selectsmart.com more specifically http://www.selectsmart.com/finalfantasy.html for FF selectors.

The ones that say "With results graphics" have those images that you can put in your sig and stuff =)

Also, if you're looking for a specific one that you have seen in somebody's sig you should usually be able to click on the banner to go to the quiz.