I forget which board you have (Carve 142/144 if my memory serves me at all).
So, there are a couple of different "scenarios" here.
If your board has a std. Tuttle fin box in it, you already have a std. Tuttle fin supplied with your board. So, if your new Select Fin is also a std. Tuttle then your existing fin bolts (6mm x 1.0 mm thread size) should work just fine. You may need to turn the brass barrel nuts in your new fin slightly to align correctly between center of the barrel nuts and the fin screw holes in the deck of your board.
The other "scenario" would be if your board has a deep Tuttle fin box, and the original OEM fin was a deep Tuttle. If your new fin is also a deep Tuttle, then no problems, but if your new fin is a std. depth Tuttle then the fin screws would be about 0.700" (3/4 ") short and you would need longer screws to reach the barrel nuts.
You can get more of the sealing washers at a hardware store (they will need to have a 6 mm (or 1/4") center hole. Try to find some "fibre" washers as they actually work and seal quite a bit better than the soft/ low durometer rubber washers that came with your board.
Yo can purchase stainless steel (SS) washers at the same hardware store or ship's chanldery.
Hope this helps,