Dish soap can help to reduce surface tension. Add a drop or two of something like Dawn to your wash cup and try painting with that. I've tried it with mixed results; there's less surface tension, so less streaking of paint overall, but the paint certainly doesn't flow nearly as well as using flow-aid. Future floor polish is a commonly used additive for washes. I have not tried this myself, but there are many articles on the web about it. The product is now called Pledge with Future Shine in USA (I think).
Not sure on a replacement for matte medium. Acrylic mediums are really like the body of the paint, just without pigment. Mod Podge could be used as a replacement maybe, and it's a little cheaper than acrylic medium. If you were desperate, you could try something like diluted PVA glue. I have never tried this, not sure if that would even work. I wouldn't recommend it on miniatures, but perhaps on a terrain project. YMMV.
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