Steve, Merc part number 27-814658 1 is the 1mm thick head gasket and are around $46.00 each.
The same 1mm thick head gasket in Pro Marine is part number 844R1 and they are $32.00 each.
Either are the right ones for your 260 2.5 Mercury and you can get them thinner or thicker if you want... I would stay with the 1mm ones.