I've seen a few small icons that look like checks but I don't even think there's a universal symbol for a credit card... just visa, mc, etc. logos.
I've seen some icons in my Art Explosion 600,000 clip-art collection. If you are a web developer or graphic designer it's a great investment. It has a ton of good clipart and photos you can either use stock or edit in Illustrator / Photoshop to make custom buttons, logos, etc.
I just want to add that just taking icons off someone else's site could be a copyright violation if they created the icons. Be very careful when using images off another website. Make sure you have the right to use the images / photography.