import tkinter.font as font
#create Font object
myFont = font.Font(family='Helvetica')
button = Button(parent, font=myFont)
#or
button = Button(parent)
button['font'] = myFont
import tkinter.font as TkFont
font = tkFont.Font ( option, ... )
# Exaple
helv36 = tkFont.Font(family="Helvetica",size=36,weight="bold")