Where to file a lawsuit.?
That depends. Most attorneys and even most paralegals should be able to answer that question for you., it's just that paralegals can't advise a course of action or give you legal counsel.
If the customer has breached the contract by not rendering the consideration due for your services provided, then you might want to seek an attorney.
First, though, I would ask him what the specifics of his dissatisfaction are. If it is genuine, you should be able to remedy the dissatisfaction. If it is fraudulent, then you may have to explore your legal options.
Let the courts be your last resort if you want to retain a good business image.
You should not have to file in Oregon.
You should be able to file in Federal Court if he is from Oregon and you are from Nebraska (true diversity) and the amount exceeds $75,000. I think you should be able to file in a venue in your home town, since the customer came there to do business with you.
Again, a proper attorney would be the one to ask if you want sure answers.