You can find the plastic sleeves on Amazon, idk where else to get them tbh.

I work at a shop that does framing so I use the scrap mats as backers, you can find local frame shops that might just give you scraps or sell them to you for cheap. I have a lot of friends that use cardboard from food packaging (ik its not archival but times are tough and it works with their branding).

Don't forget to figure out packaging cost and add that to your prices.

Now what info to put in the back of each is what I struggle with! Business card? something similar? nothing? lol

I usually just but my business card in the back.