I've taken one Copics class from a long-time certified instructor, but am not a colorer. IOW, I don't color pages or large amounts, just use coloring mediums here and there. So my experience is limited.
So far I've used Neenah Classic Crest (not Exact Index) in Solar White and been happy, and I've loved Copic's X-Press It Belnding Cardstock, which was used in the class and was my instructor's fave. It's super-smooth. It's spendy so I use it for special occasions.
https://imaginationinternationalinc....25-sheet-pack/
Someone I know likes Georgia Pacific. And the instructor also likes The Paper Cut's White Luster or Cream Luster. You can buy any quantity to try out, even one sheet, and they attend Stamp & SB Expos. Caveat: their search function isn't great, but they'll help you find what you need.
www.thepapercut.com
ETA You can also buy pre-cut tags and various shapes in white or cream luster.