On Bedrock, there is the founder's cape which is free, and you get another cape for playing the beta version. You can also watch a video on how to get custom capes and they will show you the process.
On Java, there is this mod called Advanced cape mod, which you have to go to a website (I don't know the name) which there you select the cape you want to use and copy the image address and paste it onto the mod. I think it's the image address, not too sure.