I am sure the wiki has more info but I am pretty sure I have gotten love potions from the makeup sales person, from the coffee girl if you go the make up salesperson route, and then you can get a few from the store, you can also get them from the club rewards I think.
For transformation potions you are stuck with just club rewards or the first dark arts store visit but then you don't get the will tamer.
I believe you are right with only one option for a will tamer (unless you choose to not get it I am guessing you can get one from the shop in that case).
Honestly the most important piece of advice I think you can have for the game is that you can drink multiple cups of coffee. So if you ever get below the energy required for the weekend event you can drink up some coffee. Also figuring out what events you can do during a day with an "end day" still rewarding the bonus energy for "resting".
Another big one is you can meet with Lauren in her office, and then meet another client on the same day. This becomes important when you have 4 clients during the transition period after finishing the Chubby girl and getting the cheater / good girlfriend.