Where to buy an effective decongestant for a headcold in Taiwan?

After surviving through the Sydney winter without catching a cold, I woke up on the morning of my flight to Taiwan with the beginnings of one.

Now I find myself in 30+ temperatures with a blocked and runny nose and watering eyes.

I'm careful with cold remedies as many are full of all kinds of things, containing both a cough suppressant and an expectorant is common enough for instance. So the only thing I ever buy is a decongestant. I don't want a medicine with any other active ingredients. With just a decongestant I become much more functional until the cold has run its course.

In Australia I always used to buy one containing Pseudoephedrine because it works best. I understand that this is classified as an illegal drug in many countries. I have no idea its status in Taiwan. I also don't know which decongestants are considered next best after it.

In Australia I would go to a pharmacy for actual medicine. Pharmacies must have a qualified pharmacist and can sell medicines that require a prescription etc that regular shops cannot sell. But other countries have different traditions and might confuse a real pharmacy with something more like an American drugstore. They might sell things like shampoo and cough lollies but not real medicine.

There is a chain in much of Asia called "Watsons" - it seems to call itself a pharmacy but I'm not sure if this means that sell real medicine like a pharmacy in Australia.

So can I buy a decent decongestant at a Watsons and if not, what kind of shop should I find and what are they called in Chinese? I prefer if it's a shop that sells one containing Pseudoephedrine if it's allowed in Taiwan. But if not, something with another active ingredient that actually works.