2005 Prius battery hybrid
There is no such thing as a cheap battery, try here.....http://greentecauto.com/product-tag/prius-2004-2009
In my area there have been some on Craigslist, people have totaled their car and selling off parts. Also try a wrecking yard, you might get lucky.
As the other user posted, Craigslist is good. I've seen offers to rebuild defective batteries for $699. Although, your Prius probably only needs a cell or 2 replaced. These you can find for about $40 each on Ebay. I had a check engine light, red triangle, and another light come on in my 2006 Prius. Had it checked and it was showing codes P1310, C1259, C1260, and a couple more. It turned out to just need 2 cells replaced. Less than 3 hours and a YouTube video later, it was all fixed. Most of the time was spent unscrewing everything to get to the battery. The actual swapping of the cells only took about 15 minutes.