Well, I wouldn't be so certain about all those moist conditions being good places to use these "cards". Enough humidity and the tiles will be too damp, adhering to each other more if they are not clean, or conversely, becoming too slippery to the touch. Additionally, accidental exposure to sand can scratch the surfaces badly enough to make them unattractive or even non-functioning for the purpose.
Lightweight aluminum might be better, possibly titanium, but titanium's pretty expensive. They're not as soft as plastic. In fact, it would be good to rough the surface and glue a playing card down to each face and back, providing a more playing card-like feel to the slabs. You could also put striations on the edges to replicate the feel of a stack of cards.
No matter how you slice it, though, twenty bucks or more for a stack of acrylic slabs cut to the dimensions of a pack of cards is a lot of cash, especially when a pack of cards can be had for so little by comparison. They have the "oh, gee whiz" factor going for them, but that wears off quickly enough. Worse, some might argue, if the weight doesn't match an equivalent stack of cards, practicing with them may throw your technique off rather than improving it.