The best deals are from MisterAuto. Very reliable.
Personally, I would recommend Walker parts, as these are the same as you would get from Renault these days - at about a quarter of the cost. If it's an 845cc variant then the front underwing box is £21.90 and if it's the F6 van type where the exit is vertically downwards, the it's £24.05 - all plus postage for about £3.99. I've stocked up on the rear silencers for my later TL because Renault want £112.00 for the IDENTICAL Walker part, with their name stamped on it - the rear box is £20.38 from MisterAuto!
I've had bits from Der Franzose and Melun and they are both superb companies to deal with, but their exhausts are, I'm afraid expensive and not as good as original. Go with MisterAuto.co.uk and you will be happy, the Walker system on my TL sounds great - perhaps even quieter than the genuine original!
Incidentally, Renault still supply all of the later system with the rear box and some of the pipe work is actually cheaper from them - just not the box!
Let us know what you decide!