I agree with all of those who said to report the dealer.
Regardless of where the tractor is bought, or for what reasons (internet shopper or not) a licensed kubota dealer cannot just deny warrenty work because the tractor was not purchased at his dealer.
They can be a PITA to work with in this situation. Like moving valued customers ahead of you, and taking their time, but they cannot just deny the warrenty. Call the reigonal rep and definatally keep us posted on the outcome.
And it seems pretty clear that the problem with the tractor wasnt the cause for denial, since you havent even told him what was wrong yet. But out of curiosity, what symptoms/problems are you having that you suspect the voltage regulator?
And how did the conversation with the dealer go??? Something like this.....
"Mr. Dealer, I have a kubota Bxxx that is under warrenty and I am having some problems and what you to take a look at it"
"Mr. Owner, Where did you buy it at? .....Oh, well we cannot honor the warrenty because you didnt buy here....click"
If it went something like that, that is just stupid. Contact local rep immediatally, or e-mail them with the details and leave your contact info.
And dont be afraid to give this dealers name.