chandlers usually have it on rolls, for small amounts it should not dent the wallet too much. For larger quantities, maybe a haberdashery or one of those material stalls on the market, maybe?
The rear of seats on (some) trains have standard cord netting - with elastic cord around the periphery. This idea seems to work very well - the elastic is easily replced when it degrades - and may be worth your consideration.
I find the easiest way is to get some guardrail netting from a chanlers, fix fairly loosely, then thread a bungee cord through the edge. I have made up a lot of pockets for cabin sides that way. Fiddly but effective.
It is available, but I also dont know a supplier at the moment! I'll try and find one, it was fitted in some ford cars, in the boot, to hold stuff down.