Cheap plastic knifes from pound shop with the retractable blades that extend enough to cut all the way through the foam
Evil Bay item 221171545196 sell four for a pound in my local pound shop
I bought a sheet of packing foam (the hlb stuff) and found that works really well for making your own inserts. Have heard that kaizen foam cuts well also.
You can get replacement pick and pluck foam from Peli UK.
https://peliproducts.co.uk/1500-pick-n-pluck-foam
Yes, I know its an old thread, but its also available for other cases.