Hi Sharon,
I have been using primarily the Wallis Archival Sanded Paper, manufactured by Kitty Wallis. You can get it at most large art supply stores, catalogues or through Kitty at 1-800-760-7870.
It is incredibly durable, and accepts underpainting washes in watercolor, acetone, mineral spirit, acrylic or thinned-down oil without buckling or filling the tooth. It's good for painters who, like me, have a pretty heavy hand.
I know of painters who literally wash off a failed pastel with a toothbrush and running water.
If I am working large (about 32" x 40" or larger, I have the Wallis paper tissue mounted to 4-ply rag, and the rag tissue mounted to 1/2" foamcore (Gator board would work well here too.) Even very large sizes are lightweight. I have had a ittle warping occasionally, but not enough to distrurb the surface shape in a frame. I would probably have something mounted on the backside next time to equalize things.
The paper comes in sheets from 9" x 12" to 24" x 36" (I just checked
Dick Blick for example, and the museum grade is $18.68), and in big rolls that measure 46" x 10 yards, or by 50 yards. It's available in Museum or Professional grade. In my view, there's not enough price difference to justify not getting the museum grade paper.
The original papers have sll been white, but it's now being offered in a neutral toned option, similar to a La Carte light grey (warm grey).