Where can you get the nectar bottles without making nectar?

You can find them by harvesting/searching prairie grass. Having some on your property via placing or using the lot challenge makes it easier to find.

You can find various nectars and other collectibles (metals, gems, fossils, frogs, time capsules, normal produce and toys) in these so they are literally respawning loot boxes. Vitality nectar goes for over 17k per bottle when it's Finely Aged and can be found.

Dreadhorse Caverns are great if you want perfect produce for your garden.