1 What follows is personal view only - feel free to ignore
2 I had one of those ionisers once - believed the hype - all I got was a grubby wall.
3 My current view is that these devices are more hype than substance. Complete waste of money, unless believing is what matters.
4 If you still believe, then wire is wire. Any electrical wire will carry the small current required. Use anything handy, but NOT nylon wire LOL. Nylon is an insulator LOL.
5 For this job, any wire of any diameter will do. As long as it conducts. Just use anything cut to the same length as the original, but even length is not critical. Look for anything that looks similar to the original (i.e. replace insulated wire with insulated, or uninsulated with uninsulated)
IMHO