You will find the best covers on line. Often covers in the stores you mention are not made of breathable plastic. If you are after a plastic type cover to go over a flat nappy, i would recommend PUL, as it is waterproof and breathable. Ones i have tried that are great include totsbots, motherease riki, bambinoneo.
I have also found fleece covers great. Many people swear by wool too.

Some stores you could try online include;
WAHMNAPS
theweewuns
thenappybucket
babysoftlandings
darlingsdownunder
just to name a few!

You might have to do a bit of googling.