American Science & Surplus has five sizes of small bags from 1"x1" to 3"x4". I don't know the thickness, but they are fairly sturdy -- much heavier and they would be difficult to manipulate. They come by 100, but they're so cheap that you would be wasting time trying to find smaller amounts.

American Science & Surplus plastic bags

(If this link doesn't go directly to the bags, just type 'zip bags' in the very upper right search box and they will come up.

Sue