These cards are on the same level as the 6570 (Assuming yours is GDDR5). If you can find any of these you will get the same performance.
HD 2900 XT, HD 3870, HD 5570 (GDDR5)
8800 GTS 640 MB, 9600 GT, GT 240 (GDDR5)
Slightly better;
HD 4830, HD 5670, HD 6670 (GDDR5), HD 7730 (GDDR5)
8800 GT 512 MB, 9800 GT, GT 545 (DDR3), GT 640 (DDR3)
Slightly worse;
HD 3850 512 MB, HD 4670, HD 5570 (DDR3), HD 6570 (DDR3), HD 6670 (DDR3), HD 7730 (DDR3)
8800 GS, 9600 GSO, GT 240 (DDR3)