I've bought Cachet papers at B&H and Freestyle (www.freestylecamera.com). Freestyle has a better selection in stock.

I've bought old Cachet photo linen for cheap from J.B. Hunt, and it does tend to show more fog than other papers if it's old. I'm not sure if Expo has the same problem.

Some Cachet papers don't respond well to certain soft or warmtone developers (like Selectol Soft and Neutol WA), so try a different developer with the stock you have. If you're getting high base fog, you can also add benzotriazole as Tom recommends and/or increase your exposure and reduce your development time, preferably to under 1 minute.