Try German Etching by Hahnemuhle, although it's pretty smooth and similar to a lot of others.
Schoellershammer Velvet has a smoother surface while still being a 100% rag cotton stock.
My favorite and by far the most popular that we've sold in the past was Hahnemuhle Torchon. It has a pretty heavy texture to it, but it's easy to print with an excellent white point.
Anybody reproducing watercolors would pick the torchon 99% of the time. Photographers liked the velvet better for it's smooth surface and the german etching, while popular at other shops, never sold well for us.
You can find some good choices @ digitalartsupplies.com, but it looks like they've stopped selling the torchon.