The best card stock for game cards is the S33 premium grade card stock which has a superior smooth texture on card surface and holds well to bends and rough handling.
3 CARD GAMES by Mugames — Kickstarter
Custom 2.2X3.4 Inch Game Cards
Custom MPC Stripe Monogram Playing Cards - Ireland
S33 superior smooth card stock with black core (smooth finish)
Custom Poker Back Game Cards Personalized Playing Cards63.5
Playing Card Cardstock Material - Offset Printing with WJPC
A35 thick standard card stock
Spectrum Noir Darice Ultra Smooth 8.5 x 11-20 sheet
Game Card Stock