Where can one obtain mailing envelopes which are more sustainably sourced than usual?
The following practice is not 100% sustainable.
However, the practice is more sustainable than what a lot of people do.
Given two practices p1 and p2 we can say that practice p1 is more sustainable than p2 in the same way that given buildings we can say which building is taller than the other building (via relativistic comparison) in spite of the fact that neither building is an element of the set of all tall buildings.
Use used shipping supplies
Purchase (or make) a large box or bin for shipping supplies.
When you receive a box, or envelope, remove the item you purchased from the package you received.
Place the used empty bubble mailer, used box, and/or used packaging material inside of your bin for shipping supplies.
The mass of materials required to preserve, and store, used packaging materials is less than or equal to the mass of materials required to preserve, and store, new packaging materials.
For example, a warehouse full of used cardboard boxes has the same amount of steel framing as a warehouse full of newly manufactured boxes.
Whenever you wish to mail, or ship, an item to another person, then place your item inside of the used box or used envelope.
Addendum on Tape
My main point was that using used mailing and shipping products is more environmentally sustainable than using new products, relatively speaking.
The envelopes can be re-sealed with tape.
Kraft tape, which is made of of paper and water-activated adhesive, is more sustainable than tape with a plastic backing or plastic substrate.
The rate at which most vegetation grows is faster than the rate at which black petroleum oil forms from dead algae and small ocean animals.
The rate at which perennial trees (and the annual plants used to create linseed linen, grow back) is greater than the rate at which petroleum finishes forming from dead plants.
Plastic tape sourced from petroleum is less sustainable than paper or line tape sourced from perennial trees or annual plants.
In most developed countries it is possible to buy envelopes made from recycled paper. They are made in a variety of colors: brown unbleached, white, cream and black.
If you enter sustainable envelopes or envelopes made from recycled paper into an internet search engine you'll be able to find a supplier new you.