I showed my brothers gf the rubber band trick and she was able to skip using maternity pants and she got BIG.
Also try Craigslist if nothing else works.
I was going to mention yoga pants....those are the best but if you need certain material then it's hard to say. I would hit up a second hand store and just buy a couple pairs of maternity ones that you won't feel so bad about ruining.
I might try the hair tie thing. I have to be careful what material the pants are made of because I work with a giant robotic welder and I don't want any sparks igniting our melting my pants lol.
My cousin's wife taught me this, but it's been a budget-saver. Use a hair band, put it through the button hole until it doubles over and you have two loops, then pull it until it fits over the button. I'm at the point where I do this and can't even do up the zipper anymore, but I just wear a tank top and then a longer shirt over it. Similar to the rubber-band trip, but you can find really strong but stretchy hair elastics that hold up really well.
Depending on the dress code, can you hit the thrift stores? A friend of mine just got through her pregnancy spending something like $20 on maternity clothes since she didn't have any real restrictions on what she could wear... I was feeling pretty jealous of her as I just sank $$$ in to a few outfits for work (have to be business casual).
Also, do you have any friends a little bigger than you? I'm a few sizes bigger than the same friend who went to the thrift stores and was able to lend/give her a few things that fit ok until near the end.
Go to a resale shop much cheaper and who cares if they get dirty they were cheap or resale maternity online I know I couple of stores locally here in mass do online sales for resale
Are they dress pants? I used a rubber band on jeans. You might be able to do that with dress pants, just depends on how they are made.
I know i have seen some ladies keep their pants unzipped and buttoned and use a tank top that is of the stretchy material and pull it over the opening of their pants
I was gonna say get the Bella band but I guess you already have it? There is a cheaper version made by the bella band company sold at target for like $20, it made my pants last a bit longer...though now I have no choice at 34 weeks but to wear maternity pants.