I am not from the US so cannot recommend, but will profer a word of warning. I mentioned this in another post, so forgive me people if I am repeating myself.
Do be conscious of the fact that sandpaper is sand attached to paper with glue. And glues are "fugitive", and not very reliable these days. If yu want to sell work, working on sandpaper is dodgy, from a long-term point of view. As I said before, the glue on sticky paper dries out, as do envelopes.
You really cannot go wrong with a good quality pastel paper. Alternatively, you can create your own textured surfaces using marble dust and gesso painted onto board, or there are, in the UK anyway, ready-prepared gesso in pots which has a wonderful, amost-sandpaper texture - Art Spectrum from Australia is excellent.
Be aware also that sandpaper, and in fact any textured surface, will EAT your pastels.
do visit my ebay page and auctions
visit my website which has a “troubleshooter” page of helpful pastel tips and hints
Also see my posts in The Artists Marketplace here at WC!