Where to buy passport protector?

I am looking for something that I can wrap around the front and back of
the passport that doesn't cost much. The only ones I've found are expensive
leather ones but I'd like something cheaper.

When I travel near the Mediterranean I wear a moneybelt around the waist.
A passport in the moneybelt gets bent and 'steamed' until it is a curved wet
noodle--not a pretty sight. A year or so ago I went through immigration in
Chicago, and the agent stuck his thumb under the picture, which had been
raised in stark relief by being moneybelted over a summer or two, and
esseentially he destroyted the passport, and told me to get a new one.

What works for me now is to put the passport on a thin hard plastic
sheet--like any piece of old Tupperware that is absolutely flat, but still
hard, and still passport-sized--and cut that rectangular passport-shaped
slab of plastic with a very strong scissors or pruning shears. If you do
this, you will have a "backing" that will let the passport keep its shape
(in a moneybelt or anywhere) no matter what kind of wear it is exposed to.
If you need to put the passport coupled with the hard plastic frame in a
simple Zip-Loc baggie to protect it from humidity, jostling, and further
indignities, this still does not take up much additional space or weight.