Upgrade to SW gear

Does the item need to be rare/uniqe or something?
And how do i get Anvil?

It doesn't need to have potential. But if it does, the potential rank will transfer. (Not the actual lines, just the rank. e.g., if your original item was Unique, the SW item will become Unique too).

Anvils drop from the bosses on Commerci voyages. Rarely.