i remember i saw some sprite in the forum some cars,clouds and other stuffs but my opinion using others design will not help you to improve, so try to do your own arts and keep growing it the best example you can see is mario see how they started and see how its now ^^
use this is good for pixels http://makepixelart.com/free/#:) for me i never use pixels i don't know why the only game i liked and had pixels art is : Junk Jack on iphone/ipad all other games i didn't like it .
for me i use photoshop , illustrator they are good on making none pixel arts :) photoshop can do pixel art but i don't like it :P