The HD 4000 series is long out of production so the replacement card was likely itself used or refurbished. Being an Apple you can't just go to Amazon and buy a new card, you will have to get it replaced through Apple, if they will even offer a replacement as Apple would much rather sell you a $2000 new system instead.
According to this website, you might be able to upgrade your 2009 IMAC 27 from a HD4850 to a HD6970 : iMac Intel 27" (Late 2009 or Mid 2010) Graphics Card Replacement - iFixit Repair Guide
Not sure if your IMAC is the same as the one mentioned here. But if it is, you might be able to upgrade to IMAC 2011 model version gpu card. It gives you photo images on how to replace it and where to purchase the HD6970 (661-5969 27" iMac (Mid 2011) AMD Radeon HD 6970M 2GB Video Card ).
But for the price they charge you can buy a new Windows 10 PC ($900.00 for Apple GPU Card).
It says to email them your IMAC Serial number and they will tell you the correct GPUs you can use on your Apple Computer.
Thanks to elstaci and zion_black.
Apple have declared my iMac as obsolete and so they've dumped all their stocks of spare parts (they really are working hard at making money from new sales !).
I have found a supplier in Melbourne that has "refurbished" HD4850 units for A$450 which is a lot easier for me than $900 for an upgrade. With labour, I might get the iMac fixed for A$600 which is bearable. If it gets close to A$1,000 I'll look at a new iMac (sorry but, Windows has made life too unpleasant for me to consider any PC) – new iMacs might be announced on 30 October.
I just hope that the old 4850 will last longer than the new unit Apple installed in 2015.