yeah the sticker should be inside the door, and you definately need to sort it.
It could have been removed if it has had any paintwork...usually the case.
To get one is a complete arse...my mate had the same issue.
You can only get em from reno. You have to fill in a disclaimer form giving all your details. If you cannot provide the old one then renault will liase with the police( Im serious) and get it inspected and the police will have to stamp the aforementioned document before the will consider ordering one. Then, and this is the best bit....it costs £70 for the feckin sticker and usually 2 - 3 months to arrive.. they have to be ordered from france.
My advice is to get one cos anyone who knows where to look will run a mile with this plate missing.
oh and reno have to fit it, they wont just hand it over - its the cars ID.