I have used lava rocks, and while light, they're not super light, and were kind of hard to shape, although they do sort of look the part.
My current ones are foam-rubber. Just a lump of furniture foam torn to bits and then coated with a paint/PVA mix and put onto a peg and base. Now those are
really light! (and soft, so they won't damage any paintjobs)
But you asked for ready made, and there's one brand that comes to mind (besides loads of Ebay sellers flogging home made stuff): Battlefield in a Box from Galeforce 9:

Bear in mind though; these are just casts of lava rocks on a stick, plus a healthy mark-up...