I buy all my meds....a lot on line from Diamond Pharmacy in Bangkok
They seem to always have everything I need and are much cheaper than regular pharmacies, they deliver to your door within 2 days, you just send them a line message asking for what you want and how many, they will send you back the prices, you just pay online and that's it.
I normally buy 3 or4 months supply at a time
Aa an example Bio-flor prebiotic is around 350 in a pharmacy for a box, it's 250 from Diamond . Just Google them for the details