Have you tried Gore Bike Wear - I find their (non-short-short) models nicely long and as far as I know they do a women's bib short. Based on the assumption that more serious cyclists will be buying the bibs I would expect them to be on the longer leg end of the Gore spectrum.
This brings up one of my pet hates about buying online - convenient as it is. I was looking at women's shorts the other day on the Evans website - there must be 20-30 different women's shorts listed but no where does it say how long the inseam is, so you're left with a huge long list of mainly black shorts which look identical. Is it any wonder they can't shift more expensive gear, as on their little photos, a £19.99 pair of no-brand looks just as good as a £75+ pair of Assos unless you know the brand. Even the women-specific websites like Minx Girl and Girls Run Too (good place for bike gear too) tend not to give this sort of detail.