Solution of very small particles with neutral buoyancy and distinguishable to the naked eye
I think that the cheapest and fastest way is that of working on natural products, like actual tea leaves, or ground coffee (I just tried it with coffee, and it can be done):

If you want only particles with neutral buoyancy, or, if I understand well, floating in the solvent, you can use said natural products, which contain particles of different shapes and densities, and discard by fractionation the unwanted ones.

What I did:


Place the ground coffee in a glass of water and stir a lot.
Let the suspension sit. You will notice that part of the parcticles are on a top layer, some are floating in water, and some are at the bottom.
Put the glass in a sink, and gently add water to it, so that the top layer eventually falls out of the glass, together with some water
Let the suspension sit: if needed repeat the last step, otherwise go on
To discard the bottom layer, gently pour the solution in another glass, by leaving some water in the first glass: the first glass will contain the heavier bottom layer, and the second one will contain only floating parcticles
If needed, repeat the last step