Basically you cannot get the sheet for free. You have to pay for I to buy it online. The prices can be different from $6 to $10 depending on the seller. I am giving you the link from where you can get the details of the price sheet.
Value Of A Sheet Of 1976 13-cent Stamps Of The 50 States
Value Of A Sheet Of 1976 13-cent Stamps Of The 50 States