Tamarind in YEG
T&T will also have it. I've used different forms from frozen whole tamrind that has to be soaked in water and strained to paste from a jar. The paste is way easier and I can't tell a difference. There are so many styles of pad that that it's hard to focus in one one specific authentic type but I found the addition of finely chopped pickled radish helps a LOT. You can find that in the refrigerator section by the Japanese pickles. I use a couple of tablespoons per bag of noodles and it lasts forever in the fridge. It almost disappears into the noodles but it seems to add something in flavour.
You can also add tine dried shrimp but we can't tell the difference with or without it.