Tamas, the very best 24mm lens for Canon FD seems to have been the Canon nFD 24mm f/2.0; after that, the Canon 24mm f/1.4L. (Both according to user reviews/internet rumor; I do not necessarily subscribe to this view as I do not shoot lens test charts.) Third party lenses of that period were not really in the same league with the camera manufacturers' lenses, though I don't doubt there were exceptions. The very best third party lenses in the mid 1980's were Tokina AT-X, Tamron SP, and Kino Precision (Kiron, Vivitar Series 1).
Your best bet, in my opinion, is to wait for a Canon. You may have to raise the price you're willing to pay, as very wide and ultra wide lenses do not come cheap, even 30 years old.