Honestly depends in terms of what you're lookin' for. Honestly my young adults wear about the same stuff like the young adults do, since it varies what kinda sim I'm makin'. Casual fashion or maybe something more sophsicated? These are Maxis Match clothing below.
JolieBean- Has some pretty nice items and these work pretty well with elders as well too. Couple of things for males as well.
Simple Simmer- I like the casual, simple fullbody dresses here
Viiavi -From casual, to lovely dresses and wedding ones to party attire for females.
Rusty Nail- Males and females mix of different stuff and good quality clothing to suit a more mature adult sim look. Korean website so make sure you have the page translated.