Daniel Flather: I did not know that! I guess that shows how much I look at the back of a lens.
Where can one purchase gels that fit?
Any gel will fit, there is an outline on the back of the lens, can't remember exactly the measurements, but you just cut the gel with some regular scissors and then you pop it in (they are just like pieces of transparency paper). A nice thing is that the standard gel size could actually give you more than one filter at the size you need, that way if you mess one up you can just cut another one. I use a folded lens cloth to hold them in an old, small lens-filter case for portability. Much nicer then carrying around a lee big stopper, which is why I prefer them. Also they are much better in terms of color neutrality than glass because they are almost perfectly neutral, AND they are a LOT cheaper...