There are also some suggestions on Sourcing non-printed parts - General - OpenFlexure Forum. It is quite a long thread and includes other parts not just the lenses.
Objective lenses have a range of prices and performance. It is hard to make a lens where all colours are focused at the same position and where everything is in sharp focus across a wide and flat area and also where there is no darkening towards the edges of the field of view and no colour tint. The most expensive lenses are designed to get all of these things as good as possible with sharpness only limited by diffraction, lower cost lenses will not be as good in some areas. The cost also goes up rapidly if you want a long working distance between the sample and the lens. The Openflexure microscope has an automatic calibration function that compensates for dark corners and colour tint. It is also motorised with automatic slide scanning, so you can tile images together to get a wider field of view and compensate if the edges of individual images are not in focus. These functions mean that you don’t really need the more expensive microscope objectives.