I presume by better you mean stiffer? Really you need to look at what you're wanting to improve/change in the way the car handles before you start attaching thicker ARBs front or rear as they can have a quite drastic effect!!
But as paul says you can get them from ultra racing but you can also buy the ARBs off the cup racers that are a bit thicker afaik (they run slicks) from RenaultSport Wimbledon.