This isn't the most common method of getting capes, though they are found infrequently on Java Edition. Either way, here's how mods can be used to get special skins and other cosmetics.

Genshin Impact version 5.7 introduces weapon skins, a limited-time customization feature altering weapon appearances without affecting stats. These skins, themed by element and weapon type, are ...

Minecraft Steve has been added to Super Smash Bros Ultimate, and there's now a mod that lets you make use of your Java skins.

Unfortunately, for Minecraft Java Edition, there are no skins that will automatically incorporate a cape. However, in Bedrock Edition, there are some skin packs that come with capes included.