Good quality and fairly inexpensive - plus great customer service:https://www.hollywoodposterframes.com/If you are looking for super cheap (in price and quality) Michaels sells 27x40 for like $15. But you get what you pay for. Don't use it for anything you value.
Plastic sleeve and pins!
I buy all of my poster frame materials at Guiry's. They have 27x40 frame kits and they will cut on demand a piece of UV protected glass and sell acid-free backing boards. I can put together a professional quality custom frame for under $70.