I did some working for you, and following is the outcome:
(255/60/R18 VS 265/60/R18; this size seems quite reasonable to me as values are quite near to the stock)
(255/60/R18 VS 265/65/R18; this size seems a bit much and may be sincerely avoided)
If your particular and specific size is not available, then you may opt for 265/60/R18, if you are comfortable with it; else if, you can ask the dealer to source your desired size for you from Karachi.