Try getting decks made by the Expert Playing Card Company using Robusto stock. It's the heaviest, good-quality playing card stock I know of.
Alternately, have you considered using plastic cards? On the plus side, they tend to last longer than paper. On the minus side, they don't have the same rigidity, the colors tend to be a bit more faded, the variety of designs is much lower and a scratch on a card back will forever mark it in your deck, forcing you to get a replacement. One of the more affordable, decent-quality plastic decks out there would be Bicycle Prestige - they're running under $15 a pack, depending on where you buy them. If you like the material but want a more interesting card back, the Karnival Death Heads Armour Edition are made by the same company (USPC's Spanish subsidiary, Fournier) using the same plastic stock. The faces are a little boring - Fournier Jumbo Index - but it does have an interesting Ace of Spades, pair of jokers and back design, assuming you're into skulls...