You can get other batteries for your phone. Bigger ones, and larger back covers to accommodate them... try googling your phone + larger battery. You could also try ebay (I bought a 5000 mA battery with cover, for my Galaxy Note, for $10 with shipping).
Your phone may not recognize the additional battery capacity (mine doesn't, completely), so you may get an incorrect battery-drain reading... best way to see if it works is to actually use it, and measure it against the 'normal' charge life of your old battery.
Samsung makes larger batteries, if you aren't comfortable buying a no-name or generic one. Available online, at most online retailers (amazon, ebay, kijiji, etc.).