Regarding saving time in the prototyping phase, there is a concept called "White-boxing" where you use generic shapes (squares, rectangles, spheres, pyramids, etc) to represent all your in-game assets (main character, enemies, obstacles, trees, etc). A lot of the popular game engines like Unity and Unreal will be able to support these basic shapes right out of the box. Then if you have more time you can go to their asset stores and find something more suitable.
Although white-boxing can save you time, I heard on a Game Design Dojo podcast that sometimes it can be misleading. The example the developer used went along these lines: He tried white-boxing a level and it led him to believe that the level was way too big and spacious . . . but once he put in the final graphics, he found that the level had the right size and feel all along. So from that standpoint, maybe it would be better to just browse the asset store.
Personally, I think white-boxing is best for rapid testing of game mechanics, then once you feel good about that, switch over to better assets to dial in the aesthetics.