Home Depot or Lowes can cut glass to size. Glass (or acrylic) is the best option to section off an aquarium if you want to keep separate colors of the same species or prevent hybridization. Shrimp are the least likely to crawl over this. You *CAN* use a sponge, but sponges can more easily be crawled over. If they get clogged, it can be a pain to clean them out unless you had a "double divider" at the bottom with spacing.
You could always look into using the corner matten filters with glass/acrylic dividers too. Going this route, I would recommend splitting the aquarium into even sections so the corner filters will be "together". Otherwise, you could use the regular matten filters in the back and have the flow going towards the front.