If you are making cards just to play with once or twice, you can simply cut up pieces of notebook paper: that works fine! But if you are interested in playing with your cards again and again, you'll want to get index cards. You might even be like me and buy your index cards in bulk! You can find lots of inexpensive index card parks at Amazon; I am a steady consumer of both white 3x5 cards (1 cent each!) and colored 3x5 cards (2 cents each!). I'm also a believer in hole-punches and metal rings for keeping my cards in packs — I can't use rubber bands because my cats will try to eat them :-).
These are my cards playing Deep Dive solitaire: poem cards and quote cards.
Yes, I really am learning Japanese through haiku! Find out more: Books for Learning Japanese Through Haiku.