Hey! From my point of view I would say that if you are about to create a 2D adventure game it will be best if you learn to make art (or have someone else to be ready to do it for you on a regular basis), because you don't want to be restricted to the sprites you find on the internet. You want to unleash your specific ideas. Moreover, when you purchase art from different sources it is really hard to keep the project consistent.
How to make art then? I don't consider myself as an artist, I don't have much experience in drawing, but just jumping right to the job and creating as simple as possible art is a great way to start. Allow yourself to make mistakes and don't be a perfectionist. Focus on improving. If you make a mistake - acknowledge that and try to do better the next time. This is how I started Clickventure.
My main goal was to create a game... And art was one of the things I had to practice. I decided to use Inkscape for various of reasons. It allows to create vector graphic, which then I export to png files for sprites.
You must decide what kind of art you want. If you want something similar to Clickventures then I advise you to download Inkscape, learn basics and practice it. It may be kind of clunky to use and time consuming at first, but stick to it and keep practicing (there is a software like Adobe Illustrator, but it's not free). Then I would advise you to create a very small game with your own art - you will learn a ton. Your artstyle will be improving over time, just don't give up. Take rests, but never give up.
If you look at my games, you can see how the art improved - compare Clickventure: Castaway vs Clickventure: The Secret Beneath, Ep.1 - can you see the difference? - The colors, the items in the inventory, the consistency, etc. I just learnt a lot from the first game.
A player doesn't really know how much of work and effort is required to create a game and I only talked about the art. Only developers know this pain. ;)
Having said that, you should realize that creating a game is not an easy task. If you are a solo developer - you are required to have a certain skill level in several professions which requires a huge amount of time and patience. Everything has to be taken care of - the art, audio, programming, design, story, animation, publishing, etc. And then you have to handle the criticism after all of your hard work.
Here, take a look at some of my Inkscape files. I create a vector art then export as *.png file. Any object that have multiple states (like a door or a chest) must be separated.
Exported sprites:
A vector file with items (in the invetory):
And exported sprites:
