1. Piano covers have to be either transcribed into a sheet or a midi. But the mp3 of a piano cover will not work. Also sometimes piano covers are not accurate or they don't transcribe the whole song. For example, there was a free sheet made by someone for SHINee's Lucifer and a songmaker tried to make it but the sheet was missing a verse or two so they couldn't finish. But they were able to make it because they received the sheet that was bought from a website.
2. Are you talking about the website, akbobada? Because that is where I get them for $0.50 each. I also buy from Upiece. They are professionally made sheets so they're accurate. Since Yunho (another admin) will not be making songs for a year, I'm the only songmaker that buys sheets. This is how I'm able to make the latest K-Pop songs. It depends on the song if I buy it or not but if you really want the song and someone's not making it, you can also buy them yourself and you can either give the sheet to a songmaker so they can make it or try making the song yourself.
This page shows you how to sign up for an account and buy sheets on akbobada if you don't know Korean: http://jouzu.wordpress.com/2010/07/24/akbobada/
and don't worry about getting scammed or something because I haven't experienced any issue like that and I've been buying from the site for a year.