Using a blackjack strategy card (as well as video poker strategy cards) is tolerated by most casinos. Nearly every blackjack game casinos deal has an advantage over basic strategy play, so even with a strategy card and perfect play, the casino will grind its little percentage out of you, hand after hand, hour after hour, day after day. That's why they allow you to use the cards. The main concern when you use one is that you don't hold up the pace of play unreasonably. In cases where a player is taking an inordinate amount of time consulting the card, the casino may ask you to put it away.
You can usually buy basic strategy cards in gift stores for a couple bucks. They're rarely exact, but usually close enough to approach perfect basic strategy play. The most accurate basic strategy cards are those by Hal Marcus and Don Schlesinger.
