"No, but the dog can get a knife for you."
Translation:Nej, men hunden kan hente en kniv til dig.
"Få" is related to be given something by someone. So using "få" would mean that someone gave the dog a knife for you after it requested it. Which would be weird.
Hente is the act of going and getting something. Another correct answer could be "skaffe", which is to get, the way that you wanted to use "få".