All the sour cream or smietana that I have found here so far is not sour enough.
Maybe try this one:
Also the other day I bought smietanka by mistake, no idea what that was but seemed to be watery, not thick like smietana.
You might have bought the one people use for coffee:
Btw, "śmietana" is sour, "śmietanka" is sweet.
"Śmietanka" is usually used for desserts:
Those with the greatest fat content are usually called "śmienka kremówka" or "śmietanka kremowa" and "śmietanka tortowa":
maybe just add a little bit of a lemon/lime juice
Apparently people also add citric acid to make it more sour.
Maybe try this one:
Or this one, if it's still available (Śmietana szczecinecka):