I found just about the prettiest, girliest i-phone covers out there. The source was ebay for about $3.50 a piece, SHIPPING included.
The quality is fantastic. They have a slightly rubbery texture but is a hard plastic. But, best of all, the cover just looks very pretty. So far, I have not seen one at a physical retailer that I like better.
The bottom one is the one I have on my phone right now. It's more impressive in person too! There are many different designs like this to choose from. These are just 3 examples.
Just go to ebay and search : girl i-phone cover