Oh shoot..I have not researched where to get new ones. I bought them here, a year or so ago from someone who was selling them. I would look on ebay, and ask on ultimate bettas, or betta breeders. You might also look on aquabid.
Why not make a cardboard cutout of the opening, take it to your local glass shop and have them cut you a piece of plexiglass to match. It would only cost a few dollers and wouldnt distract from the simplicity of the tank. Plus it would be 24hr prevention against him jumping ship.