Go to your local plumbers supply or hardware co-op and buy some closed cell pipe insulation. Comes in lots of sizes and can be cut easily and squashed to insert via head tube or BB. Heavy by the length but you only need a few short pieces.
I was using that too.
It is possible to silence the rear brake and shifter hoses in the chainstay this way, but the dedicated foam tubes are easier.
I tried cheap foam tubes for car doors from Ali, they are ok, but much heavier.
I don't think similar light foam is available( yet
) from Ali.