Windmill wrote:

Does anyone know where to buy this chain, or whether there's a way to
join ends with something (small spheres and a crimping tool ?) which is
no larger in diameter than the chain itself ?

Ball chain is used for badges/passes that hang round people's necks in
offices etc. To make it into a continuous loop they use a little tubular
construction that traps a single ball in each end, joining it together.

On a related note it's bloody hopeless as bathplug chain. I've replaced
two lately with lengths of stainless welded loop chain (as used to be
used effectively (and long lastingly) 40 years ago.)

Scott