Jetworks Jets are made with Extruded Polystyrene sheet (XPS) which is easy enough to be found worldwide. Different manufacturers produce various ‘recipes’ of foam that result in soft grades or harder grades.
The Modelling standard was a white foam called ‘Depron’. But that has unfortunately been discontinued. Grey Depron is still available, and is perfectly usable, but it tends to be softer and scuffs/dents/marks easier than White Depron.
There are numerous other brands out there, and it is a case of looking around to see what you can find.
I recommend typing into Google