205 fits a 6.0 fine in all aspect ratios you can actually get (50-65). Biggest ive put on a 6.0 k-car is 215/60r15 and i think it looks good and works well.
The problem here is that only certain things look 'good' on a CSX. Stock 205/50r15s is one of them. IMO if you want to run a larger diameter tire, you need larger wheels and lowering to make it look right, otherwise you are just raising the car and making the wheel/tire look less aggressive.
205/50r15 is one of the cheapest tires you can get, IF you are willing to settle for the lower-end ones. If you're determined to buy used, 205/50r15 are somewhat hard to find, but Id try asking on the local enthusiast boards/forums, like a local SCCA site, or of cars that typically use that size like small hondas. Unless these tires AND the mounting balancing are near free i dont think it's worth the money to end up with something thats a downgrade from stock. Again, just my .02.