You cant really make an ecosphere without some kind of natural water body, so before making one it is necessary to find an appropriate place. Ecospheres can be made using a variety of water bodies including rivers, ponds, lakes and vernal pools. Be sure to only use water bodies far away from any roads or any other sources of pollution. It’s also a big bonus if you can find a water body that contains natural aquatic plants as you save money on buying plants.
You will usually find the most biodiversity in woodland ponds and vernal pools. This is because they tend to contain less predators allowing more vulnerable species to survive. This is especially true with a vernal pool which dries up seasonally, meaning no predator species can colonize it. Woodland ponds are also biodiverse because they have a constant source of food from leaves that fall off the trees and are also often shaded. If a pond gets too much direct sunlight, this can cause an algae bloom which may reduce biodiversity. Rivers will contain less algae as the water is constantly flowing, preventing it from building up too much. I don’t actually recommend using large fast flowing rivers because you will find a lot less biodiversity as things are constantly being washed away. The same is true for large lakes however this is due to there being more predator species which consume many of the organisms we like to use in ecospheres.
My personal pick would either a slow flowing river or stream, shaded pond (not necessarily a woodland one just protected from direct sunlight) or a vernal pool. If you want even more biodiversity, I would highly recommend using multiple water sources at once (this is not necessary at all, I just do it because I have multiple water bodies near my home).
If you were to go this route, I would suggest using water from a river as it is much less saturated with nutrients which could cause the ecosphere to crash. Along with substrate from all available water sources to pick up organisms from all the different ecosystems.
Just in case you were curious, a crash is when the ecosystem loses its ability to sustain itself due to imbalances. These imbalances can be caused by factors such as too little oxygen, too much dead matter, too many of a certain organism or too many nutrients in the water. Just about anything that upsets the careful balance within an ecosphere can cause a crash. This almost always results in a mass extinction and is unfixable.