You can buy it at any pharmacy. I buy the 5mg size. However, I have noticed it does not really have a big impact on my blood pressure. I tried the 10 mg size which does take about 5 points off my blood pressure but has unpleasant side effects. As a consequence , I use other methods and medication to bring my blood pressure down that are more effective..
( less alcohol and more exercise for starters )
One thing you will notice in Thailand is that they check blood pressure more frequently. My mother never knew hers was high until she visited our excellent dentist here and her blood pressure was checked. This was confirmed by her GP when she returned to the UK where nobody had ever bothered to check it.
Walk into any government hospital in Thailand and you will see blood pressure monitors scattered about where you can self test and get the printed result. No charge for this and no permission needed. At my local hospital ALL patients are required to self test and get the print out before seeing the doctor. Best thing is to get as many of theses readings as possible since a single test is no real measure of your average blood pressure. Go every day of your holiday if you can ! Only takes 5 minutes and nobody will bother you if you just walk in , test yourself and leave. Perfectly normal here.