One of the things I love about living in Portland is all of the local businesses. My favorite store is a little place called Collage. They have two locations but I have only been to one in the Northeast, on Alberta Street. They have a great store with all the art supplies and collage bits you would need. The stores also offer some pretty great classes and workshops. I would love to look into how to put together a class in the store as I think it would be pretty rad to teach a mail art class!
I spend entirely way too much when I go to Collage. If I could afford to buy every roll of washi tape on their display, I would be a happy dactyl. Speaking of pterodactyls, I purchased one of my favorite pterodactyl stamps there. They have such a wonderful collection for everything they offer. As long as I have the money in my budget, Collage is my go-to store for anything I need dealing with mail art.