If you’re in the US, RockyMountainATV/MC has always been my go to for the best OEM parts availability and prices for offroad bikes. Not to mention great selection of aftermarket parts as well. KLX should still get some parts support from there, likely sharing parts with the KX line. They might actually ship out of the US, but ive never tried?
My next go to is Motorsport.com, but RMATVMC almost always has the best prices and best shipping for OEM.
I played through all 3 games again back-to-back a few years ago (having previously only played separately as they released), lots of stuff tie them all together and loop back to familiar scenes and incidents from different angles and times, even a 2 part DLC for Infinite. Basically looping back to where the first game starts that kinda makes it an Infinite story loop…very well done.
Also, there’s a book that prequels the first game- tells the story of Ryan building Rapture and how it all got started. Lots of back-story and lore on quite a few characters…especially “Fontaine” and how he ended up as the fish merchant…