Hey, for the Openflexure Microscope I tend to buy the lenses from Aliexpress, usually Sunlight Optical but there are many suppliers. The mass-produced objectives are generally all RMS-threaded and designed for "finite conjugates" (they produce an image 150mm from the back of the objectives), and the cost mostly reflects the level of correction (which is related to the number of lenses inside). The cheapest "achromatic" objectives may only have one doublet lens inside (corrected for colour imaging, but not for a wide/flat field of view). These typically cost around $20-$25 and are 35mm from the "shoulder" at the top of the thread to the sample. "plan" or "semi-plan" correction requires at least one more lens inside the objective, and usually is found in 45mm high objectives, costing $25-$50. These will give a much bigger in-focus field of view.
It's possible to add in more correction, or to image at "infinite conjugates", both of which are done in modern lab microscopes - and that's why you can pay thousands, rather than tens, of dollars for an objective if you want. However, I think for the PM work, a basic plan corrected lens will do. My experience so far is that semi-plan is probably acceptable at 40x, but for 100x you really want plan correction (and you need to use immersion oil too!)