if you want to have 6581 chips, check out ebay. You can determine which chip is inside of the c64 by looking at the keyboard. if the special characters are print on the key (besides the letters) then you can be sure that a SID 8580 sits in the c64. if they are print on front of the keys, a 6581 chip will be inside the c64. or check the function-keys. if there is only F1, F3, F5, F7 is print on top of the keys and F2, F4, F6 and F8 is print on front, then you will find a 6581 inside. if F1-F8 is print on top there will be a 8580 inside. Hope I could help you out a bit ;)