All you need is:
1.) Speakers - the 6 1/2 variety
2.) Bolts - 8 - 32 fits well
3.) Nuts - make sure to get nylon lock nuts
4.) Female spade disconnects
f you have the speakers it will only cost $7.50 but if not it will be a bit more expensive.
Also, now is a good time to remove those covers for the rear speakers.