Check out the Finning Web site and find your nearest dealer. I have used Aberdeen and Glasgow as well as HQ down south. Excellent service and reasonable prices
It depends on which parts. Consumables like filters, oil, anodes etc, I sourced separately, finding commercial vehicle suppliers. Most other parts from Finnings. I always found them very helpful.
I have 3126's but obviously still CATs and I generally haven't had good experience with Finning (my dealer being Poole). My first 3 orders were a complete mess to the point that I ended up emailing Finning's M.D. To be fair to them I did get a personal phone call and apology both from him and the Marine Director but I find their prices eye watering and whether they have the ability to send out the correct part when I have the old part in my hand (so give every detail possible) remains to be seen. Not wanting to be an intentionally 'negative-nigel' but I still try to source outside of Finning.