The easiest way of sourcing organisms is by scooping up duckweed. To find things like amphipods and isopods, you can scoop some duckweed with a net and place it in a clear tub with some water. You can look through the bottom of the tub to observe what you have found and carefully pick out the organisms you want. You can make this easier by stirring the duckweed and skimming most of it off. You will be left with lots of organisms which are easier to pick out. Another excellent method for gathering organisms is to find a large piece of wood in the water and holding it over a tub and pouring water from a cup down it (with the water running off into the tub). I have found this to be a great way to gather things like planarians and snails. You can even find hydra with this method (see Hydra ). Be careful not to add too many organisms, as this could overload the ecosphere and cause it to crash.
If you were unable to find native plants, you can use these methods once you have obtained them later on. I would never recommend adding extra organisms without sufficient plant life. I also recommend adding some organisms later, once your ecosphere is established.