The std system is only available as Renault origional. That said, you do not have to go via Renault dealer.
I had mine go about a month ago, took can to local tyre/exhaust place that i've used for years. THey had to get via Renault network - as apparantly none of the patern companies are making 172 systems.
The 172 system as factory fitted is single piece - CAT back. However, the replacement has to be 2 piece as it isn't possible to fit it single - it is fitted at a fairly early stage of car build apparantly. Mine cost me £325, apparantly it is much dearer if you go direct to Renault dealer.
You can get cheaper aftermarket stainless as has been mentioned. I needed my car back on the road as quite as possible, so i just had to go for std as they could get it next day.