I did buy mine off amazon… But the Capacitive soil moisture sensor for arduinos you can find in many online electronic stores. If you are looking for some sensor that is already build and that you don’t have to interface with an arduino, I don’t have any recommendation, sorry.

If you are interested in potted house plants, my current opinion is that weight is the best way to figure out the moisture content, because there is a very noticeable difference between dry and wet substrate. So I walk around the apartment every day or two and give a slight push or pull to each pot to determine whether they need to be watered. It took some time and some dead plants to get into this habit, but by now it is almost automatic.