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 Town Hall in Upper St. is where I've always got them from.
Haringey online ordering service is easy and quick, but nb the online payment system only seems to work during office hours.
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