This is a good reason why it is important to include your location in your profile. We have absolutly no Idea where in the US you are located. It makes it kinda difficult to reccomend a good place for parts if ya know what I mean.
And we also don't know what model tractor you have either. An other important peice of information that is missing here.
But basically, the only thing I buy from kubota is the hydraulic and air filters. The kubota hydraulic filter is well worth the $30. Becasue even good aftermarket filters like hastings, baldwin, and dolaldson cost that much. And the kubota filters have the circular magnet inside. And the airfilters are unique to kubota.
But on most models, the oil filter can easily be crossed to any major brand. NAPA, wix, fram, purolator, stp, etc. Take your pick. And the oil can be bought at any parts store. For hydraulic oil, some perfer the SUDT. I personally have not had any problems running the Traveller universal stuff sold at TSC for ~$30 per 5 gallon pail.