I've been using it for 3 years now, and it works like a charm.
Check the garden centres or hydroponics stores. It's used for aquatic gardens, ponds, and landscaping.
Turface is another product, same stuff but cheaper (meant for sports fields).
Profile Aquatic Plant Soil a.k.a. ceramic soil
You can find the link for Turface there too.
As the web page says, it's "Inert, neutral, does not leach or break down". It's meant to be a soil additive, specially in clay soil, or in ponds, since it has millions of microscopic pores for beneficial bacteria to hole up in and break down the soil, etc. etc. etc. However, for incubation, those pores work wonders for holding moisture, and the irregular spaces between the granules make for fantastic air exchange with the egg, and lessen the chance of drowning the embryo with soggy