Good luck finding a replacement in the hardware store or even through the original manufacturer. Times like this call for a little ingenuity and craftiness. Enter the Cookie-Cutter Sugru Method.
What You Need:
2) Wax Paper (optional, but it helps the Sugru not to touch food-prep surfaces)
3) Rolling Pin
4) Toothpicks or Kebab Sticks (same thickness as you intend your seal to be)
5) Cookie Cutters or Scissors
Step 1: Preparation
Place them on a flat surface like a cutting board.
Step 2: Forming & Curing
Place the Sugru between the two sheets of wax paper and then on either side, place two toothpicks or kebab skewers (anything which is the same thickness as your intended rubber seal will work). The idea here is that when you pass over the Sugru with your rolling pin, the toothpicks will let the rolling pin form an even thickness across the entire surface of the Sugru.
Now, let the Sugru cure over night. Sugru recommends a full 24 hours curing time.