A dollmaker or Picrew (which is actually the name of a specific platform) are services where you can create custom characters, icons, and portraits that have been created by other artists just like yourself! These services are free to use and can be found everywhere. They're great for making OCs as well!
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Here are some free to use platforms:

When using these sites be sure to properly link the page and/or credit creator of the custom image along with properly reading their terms of service to see what is and is not allowed. Some creatives do not allow for their images to be used for OCs and it's good to always double check!
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You could even make your own custom avatar creator too! A lot of these services do not require any sort of programming and are fairly straightforward!