Roselia spawns at
Axew's Eye, South Lake Miloch, Giant's Mirror, Dusty Bowl, Rolling Fields
Bridge Field, Dappled Grove, East Lake Axewell, Giant's Cap, Giant's Mirror, Hammerlocke Hills, Motostoke Riverbank, North Lake Miloch, Stony Wilderness (Max Raid Battle)
so I think it's safe to assume that it's fairly easy to find in the wild.
As for encounter rates:
Axew's Eye - 40% encounter rate as a Random Encounter while it's Cloudy.
South Lake Miloch - 30% encounter rate as a random encounter when it's Cloudy, and 20% in Harsh Sunlight.
Giant's Mirror - 20% encounter rate as a random encounter when it's Cloudy.
Dusty Bowl - 10% encounter rate as a random encounter when it's Cloudy.
Rolling Fields - wanderer, found in normal weather, Cloudy weather, Rain, Snow, Harsh Sunlight, a Sandstorm, and in Fog.
Bridge Field - can be found in a Raid Den, with a 0% spawn rate in 1 star raids, 20% spawn rate in 2 star raids, 40% spawn rate in 3 star raids, and it can't be found in any other level.
I'll stop there because I think you get the point. Roselia is fairly easy to catch in the wild. In my opinion, that's the method you should use.
Hope this helps! :)