Living roof drainage

Just wondering what types if drainage folks are using under the living layer, other than heavy gravel. Looked into pumice gravel but it’s very expensive. Drainage mats aren’t locally available. There’s a product at local big box store like very heavy duty landscape fabric that made me wonder if recycled indoor outdoor carpet might work.
Any ideas or suggestions? This is on a new approximately 20’ diameter octagon. Roof is presently covered with ( temporary) selfstick ice and water guard and tarp. Thanks

Here are some details;
Green roof drainage

Thanks but that really contains no useful information other than “ a commercial product may be available “..  
appreciate the try though. Tom

I have seen that bubble/ drainage material used in fruit or vegetable boxes.
Plastic egg cartons
Also it talked of gravel.