You can find Bicycle playing cards at any drugstore, mass retailer, or convenience store. Check the toy aisle.
USPCC (US Playing Card Company) makes all sorts of decks. A lot of them are limited run and really awesome, too.
http://www.bicyclecards.com/
Amazon has all sorts of playing cards, too, that are NOT USPCC. I've gotten Hello Kitty ones there and from the dollar bins at Target, oddly enough. So if you're at Target, Wal-Mart, etc. check out the dollar/discount/clearance bins, too.
I collect playing cards and am always looking for new and unusual ones. Look around in secondhand shops and places like yard sales, too. I got some plastic playing cards from my neighbour for 50c that they sell at Phoenix Art Museum for $10 a pop, so definitely look around.
Playing card cartomancy is a bit different than Tarot. A lot of the meanings do overlap, though.
Good luck and happy hunting!