I collect alot of publications - many oversized. I really like
http://www.bcemylar.com/ - especially the 4 mil mylar sleeves. They come in all sizes. They are excellent archival material.
Then - while not necessary, I go to E. Gerber and buy specially cut-to-size acid free backing boards. This gives the rigidity you want for better display. I do want to emphasize that everything must be acid-free in your solution. You don't want a regular cardboard backing in an acid-free mylar - sort of defeats the purpose.
I hope this helps.
When you are done, you will have a really nice item to display.