It is often done locally. I have done it a few times with mixed results. I find I don't need to know what the plant is but do need to take some extra care. One is having a separate tank to put the plants in until I find out more about what I have. The first step in collecting for me is finding the right spot. It needs to be a place which is not protected by rules, etc. Then it also has to be a place where I will find true water plants. Not plants that happen to be in water because of rain!
When I get them home and in a tank, I find all kinds of things that surprise me. Nature comes in lots of forms, most of them safe but still a surprise. Some live, some die but it is all still a part of the fun. Most of us need more cheap exercise and this is an excuse to get out and waddle around in the mud!