Using your description of "four flap envelopes", I used google.com search engine and found this web site:
http://www.conservation-by-design.co...n/photon4.html
You may want to ask your current supplier what the "real" name is of these envelopes, then use that term at a search engine to see what you come up with. These four flap envelopes look to be what you're describing, but the place is in England, so maybe we in America have a different term for these envelopes.
It does look, by the other items the search turned up, that these are geared towards the photo industry. So maybe also asking a local professional grade photo house would be helpful.
Hope this helps!
Linda