If you are prototyping a card game then there's no need for super high quality materials. You want to use some kind of cardstock from your local art supply store. Try to get 110 pound index stock if you can. That's the thickest that will easily fit through most low-end inkjet printers. If you are using an inkjet printer, you'll want low-priced inks. Try to find a discount ink vendor locally or online so you do not break the bank just by buying ink.
Now if you need a colored border for your cards, since these are just protypes, do not waste color ink. Buy colored card stock. Lots of stores only sell 65 pound colored stock, and that's a little flimsy, but functional. You won't be able to print full color icons very well on colored stock, but you can print on Avery 05420 Multi-Use Labels, and they are just the right size to produce two spot color icons when cut in half. This is a method to give you cards with full color borders and full color icons, for relatively little color ink.
When you print them, realize that you can easily fit eight 2.5" x 3.5" cards on a sheet of 8.5" x 11" letter-sized stock. You can actually fit nine such cards on the same sheet if your printer will print closer to the edge; some EPSON printers don't normally print close enough to the edge for nine cards to a page, but have a feature in the printer setup control panel called "Extended Margin" which allows for nine cards.
Next comes cutting them out. If you are producing just one set, use a pair of scissors. If you are going to purchase a lot, get your hands on a Dahle rotary trimmer. It's great, relatively inexpensive if you are going to use it a lot, fast, and cuts nice straight lines. You can, if you want, give the cards uncut to the playtesters, but you'll get a lot more playtesting done if you give them the cards cut.
If you are going to do a lot of cards, even a Dahle rotary trimmer is gonna get real old, real fast. You'll want to read the glossary entry on microperforation to get sources on microperforated card blanks or card stock.
If your cards don't come out perfectly the same size, don't worry. Every card game designer needs a large supply of card sleeves. Card sleeves will add a little thickness to your cards, but if it's not enough, you can easily take another playing card and stuff it in the sleeve behind your prototype cards.