I don't know if you are still in search of these or not, but I remember seeing them at the close-out store "Big Lots" if you have that nearby. My girls have had some but I just don't think the plastic is durable enough for us. If I remember correctly, I think Big Lots sold them for around a dollar. Actually you might even want to check dollar stores too, I could see them carrying them too (although its just a hunch)

HTH