I have always enjoyed collecting my own supplements for the tanks and tubs.
Over the summer I have collected a large amount of Asellus, Daphnia and Lumbriculus sp. from a small pond at my parents' place. The pond is around 30x30 metres with overhanging Alder-trees and a thick layer of branches and leaves over a clay substrate. The water is only 50 cm. at the deepest place. No fish are present.
Easy to spot in the margins!
In the warmer months big clumps of Lumbriculus can be spotted along the edges and collected with a small net. Of course a lot of leaves, mud and small leeches etc. is collected along with the worms and to get the worms "purified", I use this simple method:
1: Pour the mud, leaves etc. through a mesh box for pond plants or something similar. Put the box into a tub with fresh, oxygen-rich water.
One of the places I collected plants this year was a small pond hidden in some trees in the middle of a suburban area in my town. Big bushes of Callithiche sp. (Starworts) and Fontinalis sp. (Water moss) ready to be taken home and de-leeched. I don't mind Asellus and other small critters but big leeches I would rahter be without in my tubs and tanks.
Apart from the mentioned, there's almost no limit to what plants you can take home and try. Just avoid the red-listed ones!
Lemna trisulca: nice duckweed!
I use them "raw" so to speak; just dried for a day and then I feed them. Stem and leaves. The whole lot! I am shure they are collected in a pesticide-free place and as a side-bonus the Poecilia wingei love the small spiders and other critters that goes in the tank along with the nettles.
A trip in the forest revealed some nice wood-peices to carry home...just make shure it's safe types like Oak, Beech etc.