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Two of the more popular but more expensive products are coconut
fibre and rockwool:

Coconut fibre, or coir, is becoming a very popular growing medium in the world. It is an
“organic” medium that offers great performance in hydroponic systems. It is the pulverized
husks of coconuts, a waste product of the coconut industry. The main advantages of
coconut fibre are its oxygen and water-holding abilities. It can maintain a larger oxygen
capacity than rockwool yet also has superior water-holding ability. Some research has
also shown that coir might have insect-repelling abilities. High-quality coir also has the
advantage of not containing any, or extremely low, levels of nutrients, so it won’t alter
the composition of the nutrient solution. It is also does decompose, unlike rockwool.

Rockwool is made from melting basalt and slag together, then spinning it to create fibres.
It has the texture of insulation and provides roots with a good balance of water and oxygen.
Rockwool is suitable for plants of all sizes, and considered to be the best substrate for
hydroponics. Since rockwool exhibits a slow, steady drainage profile, the crop can be
manipulated more precisely. However, there has been a growing concern about its disposal,
as rockwool never truly decomposes.
Text by S.C (pics by Pixabay) ref: alternative growing substrates for hydroponics