ratshack carries a variation of the battery but charges like $70 or so. you used to be able to get them for really cheap ~$30 or so at some wallmarts and people have also found them around that price at CompUSA or Staples.
the last one i got was branded energizer, got it New off ebay for like $20 shipped or something like that. although that one is 7.2v, but should still work fine. i have bought three off of ebay now and I don't think i've paid over $30 for one shipped. they are commonly up there as ESA as well.
some search terms to use: esa battery, universal dvd battery, external dvd battery, li-ion dvd battery, etc.