I just wanted to tell everyone about a great source for fresnel lenses - http://3dlens.com/enter.html . They have great prices, for example 80 cents for each credit card sized lens, and volume discounts at 10, 25, 50, and so on. The quality is excellent, see below.
I reccomend the credit card size especially, because it the smallest, cheapest, comes with a protective cover(so does the pocket size), and even with the cover, is thinner than a credit card(although with the cover, it is about a centimeter longer and half a centimeter wider than a credit card). It is also more powerful than the larger ones, 4x instead of 3x, and seems just as effective for starting fires. It probably is worth getting some larger ones though, as even the largest is less than $2 and just as thin as the smallest one, so if you have a survival kit with a flat side or flat items, the lens will take up almost zero space.


In case your not familiar with these types of lenses, they are valued for survival use because they are thin, durable, and very powerful. I've never used one until I recieved my order today and it exceeded my wildest expectations. Using the credit card sized lens, leaves, my wooden deck, and anything else I put under it started smoking as soon as I focused it. I got a fire going in a minute or less using dried crumpled leaves. The same goes for larger sizes, including a large 7"x10" lens. I don't know if the larger ones where any more effective, since the size of the focused spot seemed proportional to the size of the lens, but since you have a larger "hot spot" to work with, it might be easier to get a fire going(although if you have a limited amount of dry tinder, the large lens would burn through a lot more of it). Of course this is in strong direct sunlight.