The last really slow colour neg was Ektar-25, which was discontinued I think in 1994. There are those that say it still works well but I suspect you'd have to get pretty lucky with how it was stored I think.
Not sure about your exposure calcs: full sun at 1/250 f/11 or 1/125 f/16 will be about right for ISO100 (actually it's 1/3 stop underexposed, 1/250 f/9 or 1/125 f/13 would be better), no ND required. Not sure why you would want an even slower film. If you put a 2 stop ND (is this a centre-cos^4 filter or something?) on there at those speeds, you will need an ISO400 film, not ISO25.
Edit: do you mean you want to bracket to 1/15? I don't see the point, personally. And if you do it, don't worry too much about the over-exposure; C-41 handles that very very well. Say you've got slightly dim light (sunny-11 haze) and want to do a plus-bracket. You take one shot at 1/125 f/13 and that will be about one stop underexposed. Then you take another at 1/15, which will be 2 stops overexposed. 2 over is no problem at all for C41. Or you take your second shot at about 1/15 f/19 and that will be only 1 stop over in the sunlit areas, still with no ND.
Last edited by polyglot; 07-21-2011 at 09:30 AM. Click to view previous post history.