There are three ways for you to earn more AutoFills:
1) You receive one free Autofill once three challenges you took part in end (you will see it in the 'end of challenge' notification).
2) Pressing on the "Free AutoFills" in your dashboard or 'Invite Friends' from the drop down menu at the top right corner.
This allows you to invite people to GuruShots and once they join a challenge grants you an additional AutoFill.
3) Press on the 'AutoFill' store in 'my challenges' to purchase more.