I am pretty much a noob to this game so pls dont judge me.
I’m trying to evolve a magikarp into a gyrados for that mystical discovery mission. But, the only ways I know of to obtain candy is to walk with a magikarp buddy or catch more magikarp.
Now my problem is that in my area, magikarp are really rare. Like more rare than dratini. Meaning the only way I know of is to walk with a buddy.
I dont really think I want to walk 400km so what other options could I do?
Unfortunately, aside from using rare candy, that’s it. You have to catch more Magikarp or walk with one as a buddy.
Good luck! 
There’s another way. Find Team Go Rocket grunts that have Magikarp. Catch it with Pinap berry and then purify it. It will need fewer candy to evolve.
Not that I need Magikarp candy, but I’ve never even seen a Team Rocket stop have one around here! lol
Noobs are welcome, we all started as one 
Use the pinap berries on every catch. That gives you a total of 7 candy if you send the karp to the professor.
Got an all karp rocket stop, so they are available.
Unfortunately Magikarp isn’t in Raids at the moment, but you can try Team GO Rocket Grunts, like @Branebs said.
Research: Catch 10 Pokemon gives Magikarp.
I’d add one more thing to this list, but of course that’s not always easy to do…
Trading:
Trade a Magikarp you caught/hatched (they are currently in 2km eggs) with someone else.
It’s not much, but a friend did that in addition to the other ways. I was his main trading partner for that, so I’ve never transferred a Magikarp myself back then, I traded them all to him.
Look at this in following way:
When you finish the quest and get Meltan at the end of it - you will find out that to evolve Meltan you also need 400 candies. Meltan as a buddy will give you one candy every 20km.
So you will need to walk with him for 8000 km and then another 4000 km to power him up if you want to do this.
Compared to this - you current problem of walking 400km to get Gyrados - is a simple walk in the park, right?
Just walk around with it. When you see a Karp around you, pinap berry, 7 candies. If you have some park nearby, wth a fountain, go there many times. They show up on those locations, much more than away from water. If there´s an event with “water based pokemons” go get them, but spare Pinap berries for those.
another tip: be patient.
there is no rush in completing that research. Somewhere in the near future there will be another water related event and you’ll trip over all the magikarp you’ll find. You’re not missing out if you wait some time to evolve one.
Just buddy one, and catch with pinaps if you ever find another one.
The K’s will add up before you know it.
Failing that I’ll sell you some candy.
