Giant Four Flap Envelopes?

[Julianne Lowman]

I'm looking for a source for those giant four sided white envelopes, like the ones Mill Pond Press uses. They are about 25x33 (that's a huge guess, I'm probably way off) and have four flaps that fold in, then are sealed with a gold seal. I get mine from a local framer (free) that gets prints from Mill Pond Press.

Any idea where I can get these? They are an impressive way to ship prints, and with a tissue cover inside, a nice way to ship pastels.