Amazon is offering the unlocked version of the handset in black, white, copper, and aqua green colour variants, with prices starting at around $650. The dual-SIM variant is also available. Another point worth mentioning is that the models being offered will only work with GSM carriers (for example, AT&T and T-Mobile).
Over in the UK, it is now officially available on Sony Mobile Store, which is selling the unlocked version in all the aforementioned colour variants for £499, with standard delivery taking 3-5 working days. On the other hand, Amazon UK has the black and white colour models (unlocked) listed for £495.
As you might know, the Z3+ is plagued with over-heating issues, but the good news is that Sony has finally released an update to fix the problem.