Pumpkin Pasties Can Be Found At The Following Locations:

When you are learning the Alomora spell from Hermione, win the challenge. She will give you a Pumpkin Pasty.
After getting Alomora Spell, go to Hagrid's house. Immediately before you get there, go down. On the left side is a small patch of brown dirt. Go through there (through the trees) then unlock the chest.
In The Dungeons (Snape's place), go up near where he has the bookcases. Up and over to the right is another Pumpkin Pasty.