For 1:1 you can use the 50mm f/3.5 macro (either SSC or new FD mount) with the life size adapter (essentially a 25mm extension tube). Coupled to the Bellows FL or Autobellows you can do great copystand and slide copying work. This is really the only flat field copy setup available for Canon FD. There are also some special 35mm and 20mm copy lenses for the Bellows for 16mm and 8mm copy work or high magnification macro.
If you aren't doing copy stand work then there are also 100mm and 200mm macro lenses available with much more working distance. Both will go to 1:1 without extension tubes.
If you just want to fool around with flowers and bugs, get yourself a set of Vivitar or other 3rd party extension tubes. They'll give your 50mm f/1.8 lens 1:1 or more and honestly if your subject isn't flat then a real macro lens isn't that huge an advantage anyways.