I'm not sure if this is quite what you need, but...
Somerset velvet for Epson is available as 24x30" sheets at 505 gm/m2, and Hahnemuhle photo rag is also available 24x36" at 500 gm/m2. Sheets this thickness are quite stiff, almost like poster-board, and stand up very well to handling, while the surface of both these papers is as good as it gets, unless you want something smoother. Both are reasonably archival cotton rag papers, though they do use some OBA's. Epson Ultrasmooth fine art is also available in 500 gm/m2 sheets, if you want a smoother surface and no OBA's.