If you want an achromatic doublet how about an objective lens from an optic other than a camera?
I don't know what focal length you are after, but thinking telescope / binocular / microscope - something must be in the right area.

New objectives would obviously be very expensive but for this application I would have thought a discarded objective from a repairman would probably do the trick. I have a couple of old 50 mm dia. binocular objectives that I was given by a repairman to use a close up lenses for intricate work. I suppose they could be around about 5 dioptres, since they work as magnifying glasses, so maybe f=200mm? (I'm guessing, I haven't got one here with me) Is that of any use?