you can get stainless pie weights at most kitchen stores. they are a long chain of stainless ball bearings.
I considered that until I realized the tiny hole that the wire runs through to hold the chain together. First use, no problem. Second use, what's all this black stuff inside my balls?
And as far as them sinking on their own, I have not seen this happen out of dry hopping in carboy for around 50 batches (both pellet and leaf). I just fill the carboy as much as possible and pretend it's good.