Water balloons?? Super cheap
A guy at work uses those Kinder surprise egg things you can get. He just glues the 2 halves together and has a loop of line hanging out to attach to your line..
I was thinking film cannisters filled with space invader may work. Spray them a bright colour, drill a hole through the centre.. Booyaa.. Cheap float
I’m sure there are numerous things you could get from the warehouse toy dept or similar.. Small balls etc.. Or shape some polystyrene..