I'm amazed you can't get horticultural grit. It is available in every garden centre and plant nursery here.
I think you need to take a day trip to England and collect a few good garden items!
The chipped bark sold for reptile bedding or orchids is extremely hard and dry. I agree it must eventually break down, but in this mix it lasts for at least 10 years.
I did try and use coconut husk chips at one time. They only provided good drainage for a couple of years before causing problems. I suspect coir will be similar.