A few years back, Ilford had a product, Galerie Smooth Pearl Pre-Mounted Boards, but I believe it has been discontinued. Everywhere I have searched seems to confirm that and when it is listed on a supplier's site it is indicated as "back ordered." Though it had optical whiteners and paper was RC type, it was great for posters. It worked very well in my large format Epsons and came in sizes from 8 x 10 through 20 x 24.
The only other product that I believe is still available is Epson's Enhanced Matte Poster Board which comes in 24 x 30 and 30 x 40 sizes. I still have four or five boxes on hand, though I don't really print matte finish much lately.
If you or anyone else discovers another product, I too, will be most interested as it can be quite convenient for some projects.