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.
Actually, it's probably easier to get two pair of 3D glasses and then swap the left eye of one with the right eye of the other. No trimming needed.
(Edit: come to think of it, there may be some trimming needed if the left and right glasses aren't the same shape but in that case flipping them over would probably work unless they are curved)
