Hi. This is my first post. I'm sorry, I can't answer the question on where to find Future floor polish, but I have used an alternate product called Holloway House Quick Shine.
I'm just now getting back into modeling after being idle for several years, but in the past I used Quick Shine on several models and haven't seen any problems - no yellowing after two or three years. I guess I should qualify my remarks by saying that I'm no chemist and have no idea what's in it, but I've been happy with the results. I used Quick Shine on canopies and also as a base and a protective overcoat for decals ... essentially the same as one would use Future.
The product is somewhat different than Future in that it has the look of non-fat milk (it is not clear like Future). Also, it is not quite as thick as Future, but I have never tried to use it with an airbrush (just brushed it over paint or dipped a canopy in a 35mm photo cannister filled with Quick Shine). There is a slight purplish-blue haze when applied, but it dries crystal clear.
Anyway, you can run a search for Quick Shine, there's a website that talks all about it. I have Quick Shine Floor Finish ... the one with the green cap.