I read an article once about battery quality for the 5th gen and it mentionned two main locations where you could get quality batteries however the price is not going to be cheap, the famous saying you get what you pay for.
Here are the two locations :http://www.ipodbatterydepot.com/index.asp?engine=google&ad=broadmatch2&adid=googlebroad3&gclid=CImpH1_69nbkCFa87MgodC3EATA
And
http://www.milliamp.com/battery-5th-generation-ipod-video.html
Here are the two locations :http://www.ipodbatterydepot.com/index.asp?engine=google&ad=broadmatch2&adid=googlebroad3&gclid=CImpH1_69nbkCFa87MgodC3EATA
And
http://www.milliamp.com/battery-5th-generation-ipod-video.html