I need to teach roost to a couple of mons for my team and i was wondering if i would be able to access it before battling the gym in sonata city.
Check the caves on Route 5. You might need to get postgame items to get it though, so it might not be possible for you to get it rigt now.
Yep ! you need the replacement for Waterfall (Magikarp Carpet or sthing like that)