Well, there are several options. Some common places are dedicated manhwa websites such as Lezhin Comics or Tappytoon. Also, some libraries might have digital collections of manhwa that you can access with a membership.
It depends on the context and the specific rules or circumstances within the manhwa. Sometimes it might be allowed, but other times it could be prohibited.
One challenge could be the sheer amount of chakra it requires. Controlling all elements might demand a vast amount of chakra, and Naruto may find himself quickly drained in battles. For example, if he tries to use all elements simultaneously, he could run out of chakra and be left vulnerable.
Some online manga forums might have links to 18+ manhwa, but it's important to note that accessing such content from unauthorized sources can have legal consequences. You could also look for officially licensed platforms that offer age-restricted content.
You can try some adult-oriented manga websites, but be careful as some might not be legal or safe. One option could be websites that specifically focus on adult content but make sure to check their legality and terms of use.