Yes the oil filters are pricey for these bike. I bought a bunch from the dealer when the bike was new. I have run out and need to order some more. I saw them in bulk on
Amazon for much cheaper. Search for the Suzuki tu250 filter.
For jets people say to try RockymountainATV.com and order the Mikuni N102/221 Series jets. Sigma-6 jets makes a jet kit for this bike that goes for $39 on
ebay.
I haven't really had to order too many parts for the xf250, but the timing chain broke on it and right now I am waiting to see if SSR will honor the warranty. They have made the warranty process very difficult.
Otherwise I have really enjoyed the bike. I was getting about 81mpg before the timing chain popped. My bike has about 2,500 miles and the piston looks brand new. I'm not sure what could be causing the carbon build up. Are you using 91 octane? Overheating maybe since it's lean? That may be worth starting a new thread on in the dual sport section.
It has been a fun bike.