How can can I get replacement wood siding to match the existing siding?
Best guess is that's cedar, which should be easy enough to find at a home center or cedar dealer. It won't be gray like that when you purchase it but you can leave it outside untreated for a summer or two and it should gray up nicely. There are chemical "graying" agents that you could buy if your in a hurry but I generally find letting nature take its course is better. Looks like in the second picture your substructure might need a bit of attention too. Don't cover up rot or moisture or you'll be doing this again much sooner than you'd like. Cheers