Thanks to SPQR for the link to
http://www.AtlanticProtectivePouches.com. I contacted them and they sent me a quote. Surprisingly, not that expensive for custom sizes, and they will do custom orders even in very small quantities, which is unusual. Obviously you get much better per-unit costs in larger quantities, but I was expecting either ridiculous per-unit small quantity costs or a "$100 setup fee" or something like that.
They said they are very busy right now, so roughly a 3-4 week turnaround, which isn't a problem.
Here is what they quoted me. Prices do not include shipping, but they are very flexible and will use UPS, FedEx, or USPS, whatever is most cost effective or what you prefer. Flat rate USPS priority mail was $11.30 shipping for 100 each of 4-mil and 5-mil (I want to compare the two thicknesses).
4 mil, Pouch Size: 10.38 x 4.50 in., Qty: 25, Price: $0.92 ea., a total of $23.00
4 mil, Pouch Size: 10.38 x 4.50 in., Qty: 50, Price: $0.81 ea., a total of $40.50
4 mil, Pouch Size: 10.38 x 4.50 in., Qty: 100, Price: $0.71 ea., a total of $71.00
4 mil, Pouch Size: 10.38 x 4.50 in., Qty: 200, Price: $0.63 ea., a total of $126.00
4 mil, Pouch Size: 10.38 x 4.50 in., Qty: 500, Price: $0.56 ea., a total of $280.00
4 mil, Pouch Size: 10.38 x 4.50 in., Qty: 1000, Price: $0.50 ea., a total of $500.00
5 mil, Pouch Size: 10.38 x 4.50 in., Qty: 25, Price: $1.13 ea., a total of $28.25
5 mil, Pouch Size: 10.38 x 4.50 in., Qty: 50, Price: $0.98 ea., a total of $49.00
5 mil, Pouch Size: 10.38 x 4.50 in., Qty: 100, Price: $0.85 ea., a total of $85.00
5 mil, Pouch Size: 10.38 x 4.50 in., Qty: 200, Price: $0.74 ea., a total of $148.00
5 mil, Pouch Size: 10.38 x 4.50 in., Qty: 500, Price: $0.64 ea., a total of $320.00
5 mil, Pouch Size: 10.38 x 4.50 in., Qty: 1000, Price: $0.55 ea., a total of $550.00
Color me impressed. I may have to get some for my CDVs, billheads, and stock certificates. They recognized my zipcode as apparently they do regular shipments of archival supplies to the university library here on campus where I work.