Sweet 'n Sassy Stamps sells 25 sheets of the Neenah Classic Crest Solar White for $5.00. Here's a link if you're interested:
Cardstock - Sweet 'n Sassy Stamps
I've used the Neenah, Papertrey's Paper Basics white, and Gina K's Luxury White (both 80 lb. and 120 lb. weights). The Papertrey is my least favorite because it isn't as bright of a white and it seems to absorb more of the Copic ink than the others.