Is it easy to get a taxi from Hangzhou airport to Ningbo ?

You may find taxi from Hangzhou Airport to Ningbo, but no so easy and it can be costly. The best way for you is to take the airport shuttle bus from the airport to Hangzhou Railway Station and then take bullet train from Hangzhou to Ningbo.

Are the bullet tickets sold in the railway station or elsewhere?

It is easier to buy the tickets at the railway stations, but you can also buy the trains tickets from the ticket booth in the city downtown areas.
If you want to find a ticket booth in your city, you can let me know your exact position and I can help you to find a close one to you.