I had the same problem early in pregnancy. Eventually, I just gave up searching, and resorted to those super-loose and baggy OR pants (the standard pale green/blue ones), like 5 sizes bigger than I would normally wear. The good thing is that they have a really high waist/crotch, so I can tie the drawstring around my ribs, well above my belly, and feel very non-constricted. I just wear a long maternity t-shirt with them, and am as comfortable as I can be (at work). Not very stylish or nice, but comfy and free.
Can you see if the OR at your hospital has some pairs to spare?
TIA
