Forum - GRIN Lens Purchase Sizing

Hi, I'm building a mini microscope using GRIN lenses, and we have 1.8 mm, .25 pitch lenses. But we aren't sure about how 1.8 mm and 2 mm lenses are different, and I haven't been able to find any 2.0 mm, .25 pitch GRIN lenses. Do you know where you can buy these bare lenses? And what the difference between 1.8 mm and 2.0 mm GRIN lenses is? Thank you, Ryan