You can buy argentium in the UK from Argex - they don't have an
online ordering facility, but are always very helpful on the phone.
Their Argentium is, I believe, manufactured in Italy by the current
European licence holder. It's depressing that this British innovation
isn't made in the UK.
Cooksons sold Argentium for a while, but are now offering
Reflections, which seems to be a successor to Brilliante. This is not
an alternative, as although its tarnish resistance is good, it does
not fuse or ball up in the same way as Argentium.
hope that helps