You can hack these up yourself from the "Real 3D" glasses given out at most 3D theaters.

    1. Obtain one pair of glasses
    2. Pry apart at the seam with a putty knife or small flathead
    3. Remove one of the gel lenses
    4. Flip it over and put it back in, note the areas that need to be trimmed
    5. Trim with scissors
    6. Reinsert gel, stick the plastic back together

There will be a slight bit of ugliness where the gel filter doesn't quite take up the entire cutout area in the plastic frame, but they work well.