I take Applied Nutrition marine collagen (powder). I get it from Holland and barrett.
Powder is better than capsules as it is more pure. You need to research into collagen to make sure you are getting a good quality one, eg no fillers, where the fish is sourced, or where the cows are raised, etc, so much research as not all are good quality. There's types 1, 2, 3, etc....depending on what you want from the collagen.
Hunter & gather are a good brand too, but more expensive that the one I take.
I'm not sure if I've noticed anything tbh, I think my nails are a bit stronger, though my hairdresser says my hair is thicker at the roots.
It's also best to take collagen with vitamin c.