I've been playing around with an idea for a card game for some time, and I think I've finally got the idea fairly well in place. How can I best start testing it out to refine it?
There are different stages to testing.
You need to make sure the core idea and gameplay is workable. You can do some of this by playing through a few games with just you playing for each other player.
Then you need to start creating the game. This will be an iterative process. You get the game together, and draft some instructions, and get some friends round to play it.
Take many notes! See how they understand the instructions; make notes of any oddities during play; see if they understand the game in the way you intend.
Make changes, and do this again. Do this a few times.
Eventually you'll get a game that's ready for beta testing with people who don't know you; maybe as a print and play download. Make sure people know it's a beta test, and make sure they have some good method for reporting problems they have and getting help.
You'll also want to think about asking people to try to break the game; trying to use weird strategies to win or "king making" or rushing or such.
If you think you'll have to frequently change cards you could use card sleeves instead of glue/tape. Simply print out a new version and swap. You'll probably have to add some kind of real card to the sleeve to give it stability.
As for the actual testing, just ask people to try it out with you and tell you what they think. You'll probably want to have a core group of testers that play more frequently so they can examine advanced strategies.
Once you're confident in your game you should have some blind playtesting: See how people handle the game with nothing more than the cards and the rulebook.
Buy normal playing cards (maybe more than one deck if you have a lot of cards in your game), print your special cards on normal paper (or write something on pieces of paper) and simply tape/glue them on the cards you bought. They will all look the same from the back and won't require you to make them on expensive paper.
Note: when you finally make the game, thing about cuttong the corners of the cards in rounded shape to avoid them to be destroyed too easely.
One Solution would be to start with index cards. No need to look fancy until you are sure you are happy with what you have got. (this could depend on the kind of card game. Like if you are using a game that can use standard playing cards then go with the obvious, however if you are making a trading card game or something then this solution could work for you.)
Once you have your cards and whatever extra stuff such as dice and what not that you need then gather up a few people and give them a rough overview of your game. Once you pique their interests bring them to a place where they can play, give them a set of rules. It's important that you don't play with them while they test it.
Instead of playing, keep a notebook with you and note down different details about how the players interact with your game. What makes them frustrated?, do they understand the rules?, are they enjoying themselves?, do they quit?, if so why?, and what not.
Also make the players feel that they can stop playing at any time so that they don't continue to play just out of respect for you, or out of the feeling of being trapped.
As you take notes also keep in mind the personalty of those you have playing. Are they your intended audience for your game? If so are you appealing to your audience? Also if they are not your intended audience do they find interest in your game? If so how can you maybe expand play to include their style and maybe broaden your market?
After you get these notes down be sure to look over your game and make revisions where necessary. Once revisions are made try to have a play test with the same players and see if you have improved the experience at all. Try the game with different people, and see what comes out of it.
You can purchase blank faced playing cards that are perfect for prototyping. Just write whatever text you need on the face of the card and start playing. It's a good idea to keep extras on hand for use as you need to add/remove/change the cards in your deck.
I could list a few sources, but your best bet is to check out Google Products to see what fits your needs the best.