I think video card is bad, where can I get a replacement?

editikan, there were issues with this model, " Apple has determined that some AMD Radeon HD 6970M video cards used in 27-inch iMac computers with 3.1GHz quad-core Intel Core i5 or 3.4GHz quad-core Intel Core i7 processors may fail, causing the computer’s display to appear distorted, white or blue with vertical lines, or to turn black. iMac computers with affected video cards were sold between May 2011 and October 2012.The affected iMacs contain serial numbers with the last four characters of DHJQ, DHJW, DL8Q, DNGH, DNJ9, DMW8, DPM1, DPM2, DPNV, DNY0, DRVP, DY6F, or F610." So you are looking for a AMD Radeon 6970M Video Card that was made after the "recall". Since we do not know where you are at, you should use your favorite search engine and check for Apple part number "661-5968". Hope this helps, good luck.