After experimenting for a while, I realized you can´t mix anything resembling an ECA stack (Ephedrine, Caffeine, Aspirin), which used to be very popular as a pre-workout or fat burner, with over-the-counter products.
The only over-the-counter medication containing Ephedrine is Wick MediNait, a cough syrup. Problems with that: It contains 19% Alcohol, so you will get moderately drunk to get enough Ephedrine. You need to chug at least half a bottle to get enough Ephedrine, and that might cause nausea (which can actually help for training, but it lasts for hours). A bottle costs 19.50€, so it´s not realistic to use this as a pre-workout.
Next, I looked at replacing Ephedrine with Pseudoephedrine, which, as the name implies, is almost the same compound. It is only marketed against nasal congestion, and unfortunately that´s all it does, unlike the stimulant Ephedrine. I´ve taken more than the daily recommended dose within an hour plus 400 mg Caffeine and didn´t feel anything.
However, Caffeine is still a great pre-workout, but I don´t feel the pills as much. Energy drinks and coffee are better. I recommend 200-300 mg 30-60 minutes before a workout. This is about 3 medium cups of coffee or a litre of energy drinks.
Aspirin doesn´t really boost your performance, but it can prevent blood clots, the dose recommended for that is about 0.2 – 0.5 pills. Yeah, I´m not gonna cut pills into 5 pieces.
If you are going to be doing a training session with short rest times and lots of sets, don´t take any vasoconstrictors, such as nicotine, caffeine or anything that tastes bad, because you will get ill. Use vasodilators instead, such as Citrulline or Glycerol.
For maximal strength or a workout with few sets, chug caffeine and maybe smoke or chew nicotine gum as much as you can handle safely. Tongkat Ali might also help. Cycle all vasoconstricting pre-workouts at a ratio of maximally 2 on, 1 off.