A useful item for anyone looking to turn their Feebas into a Milotic in Pokémon Sword and Shield is the prism scale. It's a small item that you need to give your Feebas and then trade that Pokémon with a friend to make it evolve into a Milotic. When it evolves, you can receive it again and add it to your collection.
To find the prism scale, you have a handful of options. The first is to locate it on Route 2. However, you cannot do this in advance. You have to wait until you receive the Rotom Bicycle upgrade to travel on the water. When you have it, go back to Route 2 and go to the professor's house and proceed west of the house. Cross the water to the island. When you are there, go northwest and you should see a distinct shimmer in the water near the other piece of land. Go over it and take your first prism scale.
You have the ability to locate the prism scale at another location, South Lake Miloch, within the wilderness area. You have to wander around the southern part of the area, looking for similar glitters in the water. It's not a guaranteed drop, but if you're constantly checking the area, you should find it after a handful of spins.
After purchasing the item, you are ready to go and you can freely exchange your Feebas with a friend of yours who will exchange it with you. We also know where you need to go to find a Feebas.