Try this:

boldStyle.configure("Bold.TButton", font = ('Sans','10','bold'))
boldButton = ttk.Button(formatBar, text = "B", width = 2, style = "Bold.Button")

Found it here.

You could of course change the font type to any type you like (if available :))

UPDATE: The New Mexico Tech tkinter website has been archived on GitHub. Also effbot was also archived since 2021.

First the best reference for Tkinter is this New Mexico Tech website. In the toc you will find a section on fonts, and in the section on Button widgets you'll find the option font.

you must have a Tkinter object to create a font

Python-2

Support for Python-2 has officially ended as of Jan 1, 2020

from Tkinter import *  # Note: UPPER case "T" in Tkinter
import tkFont
root = Tk()

Python-3

Python-3 Tk wrappers differ from Python-2

from tkinter import *  # Note: lower case "t" in tkinter
from tkinter import font as tkFont  # for convenience
root = Tk()

create a font like the example from New Mexico Tech website

helv36 = tkFont.Font(family='Helvetica', size=36, weight='bold')
# you don't have to use Helvetica or bold, this is just an example

(Note: recall for Python-3 font was imported as tkFont for convenience)

now you can set the font for button created from Button in the original post

button['font'] = helv36

The size of the button will depend on your geometry manager, EG: grid or pack. Only the grid method is covered in the layouts section by New Mexico Tech site, but effbot.org is also a great reference and he covers pack pretty well.

try:  # Python-2
    from Tkinter import *
    import tkFont
except ImportError:  # Python-3
    from tkinter import *
    from tkinter import font as tkFont
# using grid
# +------+-------------+
# | btn1 |    btn2     |
# +------+------+------+
# | btn3 | btn3 | btn4 |
# +-------------+------+
root = Tk()
# tkFont.BOLD == 'bold'
helv36 = tkFont.Font(family='Helvetica', size=36, weight=tkFont.BOLD)
btn1 = Button(text='btn1', font=helv36)
btn2 = Button(text='btn2', font=helv36)
btn3 = Button(text='btn3', font=helv36)
btn4 = Button(text='btn4', font=helv36)
btn5 = Button(text='btn5', font=helv36)
root.rowconfigure((0,1), weight=1)  # make buttons stretch when
root.columnconfigure((0,2), weight=1)  # when window is resized
btn1.grid(row=0, column=0, columnspan=1, sticky='EWNS')
btn2.grid(row=0, column=1, columnspan=2, sticky='EWNS')
btn3.grid(row=1, column=0, columnspan=1, sticky='EWNS')
btn4.grid(row=1, column=1, columnspan=1, sticky='EWNS')
btn5.grid(row=1, column=2, columnspan=1, sticky='EWNS')

Also try ttk.

There may be a simpler way to do this, but it seems to work and meets your criteria. It does this by creating a custom ttk.Button subclass that has bold text by default, so shouldn't be affected by any changes to other button styles.

import tkinter as tk
import tkinter.font as tkFont
import tkinter.ttk as ttk
from tkinter import messagebox as tkMessageBox

class BoldButton(ttk.Button):
    """ A ttk.Button style with bold text. """

    def __init__(self, master=None, **kwargs):
        STYLE_NAME = 'Bold.TButton'

        if not ttk.Style().configure(STYLE_NAME):  # need to define style?
            # create copy of default button font attributes
            button = tk.Button(None)  # dummy button from which to extract default font
            font = (tkFont.Font(font=button['font'])).actual()  # get settings dict
            font['weight'] = 'bold'  # modify setting
            font = tkFont.Font(**font)  # use modified dict to create Font
            style = ttk.Style()
            style.configure(STYLE_NAME, font=font)  # define customized Button style

        super().__init__(master, style=STYLE_NAME, **kwargs)


if __name__ == '__main__':
    class Application(tk.Frame):
        """ Sample usage of BoldButton class. """
        def __init__(self, name, master=None):
            tk.Frame.__init__(self, master)
            self.master.title(name)
            self.grid()

            self.label = ttk.Label(master, text='Launch missiles?')
            self.label.grid(column=0, row=0, columnspan=2)

            # use default Button style
            self.save_button = ttk.Button(master, text='Proceed', command=self.launch)
            self.save_button.grid(column=0, row=1)

            # use custom Button style
            self.abort_button = BoldButton(master, text='Abort', command=self.quit)
            self.abort_button.grid(column=1, row=1)

        def launch(self):
            tkMessageBox.showinfo('Success', 'Enemy destroyed!')

    tk.Tk()
    app = Application('War')
    app.mainloop()

Result (Windows 7):