Thanks for your help.

The link to surplusshed is interesting.

For this project i want to use a standard mount because of availability and reproducibility.

Edmund  optic i knew, but as you said not for low cost.

I think i will use a classic M12 or M7 / M8 mount.

For the firts sample i will use a M12 mount lens system, from Eric's link.

Ted, I really like your 3D magnetic field scanner project, it's amazing.

I've purchased cheap (surplus) lenses from these places:

http://www.surplusshed.com/ (some really good bargains here if you look hard enough.  Check out their "lens finder")

http://www.edmundoptics.com/clearance-products/ (this used to be anchor optical surplus)

For top-quality (and top-dollar), of course, you can't beat Edmund:

http://www.edmundoptics.com/ very expensive

The other thing you might consider is using a standard C- or CS- lens mount.  Then you could use a wide variety of lenses on your device.  Cheap CS-mount lenses are made for security cameras, and C-mount (same thread, different flange distance) used to be a 16mm movie camera standard, so old lenses are cheap on ebay.