Where can I find potions?

At the Hounds Pit Hub, one character in a little garage workshop set-up (right before the boat that Samuel uses to take you to the city) sells them.

And you can find them practically everywhere, you just have to look at every nook and cranny.

Under Powers, buy Dark Vision and upgrade to level II with the runes you've been collecting (use the heart you were given to find Runes during each mission, if you don't have any) then you can see if any are nearby. Otherwise, you can buy them from Piero in his workshop whenever you're at the Hounds Pit (before leaving on a mission) and I believe Griff may sell some if you saved him during mission 2. To save Griff: from the boat, go straight until you can go left up some steps, go right at the top and go to the end of the street, make a left at the end and then another left. Just before the trash dumpster at the end you'll see two guards in front of a boarded up doorway on your left. Dispatch the guards and smash the boards with your sword. Once saved, Griff stays there for a short while but eventually will go to another area. From the place you saved him, go out the door, go right until you see some steps on the left and you'll find him wandering around at the top. Also, In Slackjaw's office, you can make your own health potions (the red ones).
To replay a mission: When you first start the game, instead of clicking on CONTINUE, click on MISSIONS or during play you can click on START and select BACK TO MAIN MENU and then click on MISSIONS. Click on the Mission you want to replay and press A then press A again to replay.