Can't answer for Islington, but Haringey Council used to sell visitor parking permits from their office by Hornsey Town Hall. It was rumoured that they were stopping that, though. Otherwise you'll need to go to Wood Green or order them by post (forms on Counsil website - haringey.gov.uk).

Islington's postal service is very quick. And a lot less hassle than having to prove your identity in person.

you can only get Islingtons vouchers in person at the office in Upper St Now. Online/ post is pretty quick though...