Shopping is the first answer. You may not find just the thing you want in a small size, but you can probably, with enough work, find something suitable. Homebrew is the other answer. One of the reasons I bought a good scale and started mixing developers and fixers from scratch was that I didn't really do that much darkroom work. Putting things together from their ingredients allowed me to mix small batches right when I needed them. (The other reason for doing this was that I like to experiment.)