Just because it needs to be said:
Fluffing it is the way to go here. And I think you absolutely should allow players to do this, or something like it. It may sometimes be a little scarier, but there are lots of things you can do to make yourself look scarier, most of which are freely chosen, and have neither an associated cost nor an associated mechanical benefit. A Knowledge check should tell PCs that the witch is not a real gorgon, though (per White-haired Witch) there definitely is something unusual going on with her hair; if they fail the check and then assume, well, that’s partially meta-gaming and partially it’s a consequence of failing the check, so that’s OK. It’s not like you’re giving her special Intimidate bonuses, features, or other fear effects on the basis of her hair, it’s just description.
The “how” is a matter of backstory, not feats, items, or magic. She was born with it, cursed to have it (much like the mythological origins of Medusa!), or blessed to have it by the goblin god, or sought out a ritual to get it. She’s a different subspecies of goblin that has it naturally. Whatever. These kinds of details are good and interesting, and add to the game. You don’t need the books for them. And your players can often come up with equally-cool ideas that also improve your world and game, and that should be encouraged.
In general, DMs should almost always, and players most of the time, feel free to describe things however they like. It allows for more varied characters, cooler and more unique descriptions, reduces meta-gaming when used by the DM, and improves investment in the game when used by players. It’s a very good thing.
All that said, it’s often best to have mechanics back up description and vice versa. But I think you are going about it the wrong way; you are looking for things that explicitly give snakes for hair, and that’s just not necessary. Snakes for hair make her scarier? OK, that means your snakes for hair can be a description of anything that makes her scarier. For that matter, they can also explain how she can grapple things with her hair, per White-haired Witch. The fact that she already had that archetype is already reason enough for me to allow wild and crazy hair like this.
But if you are really blanking on ideas, here are several.
Snakes for hair could easily be a description of the Intimidate bonus from the Persuasive feat, or from any other feat that gives Intimidate bonuses, extra options for using Intimidate, or otherwise applies Fear effects.
The hair could be a masterwork “tool” (the magic item guidelines suggest that innate items that cannot be removed are between 50% to 100% more expensive than their usual cost).
Maybe the +3 in-class bonus is more because you have scary snakes-for-hair than because of your class.
Maybe instead of actually training yourself to be scarier, you just have snakes-for-hair and you put some ranks in Intimidate to reflect that.
Maybe there’s some reason in your backstory why, if you didn’t have snakes for hair, your Intimidate bonus would be less than it is, but you don’t have that hypothetical penalty because you do have snakes for hair, so that the two descriptions cancel each other out and your Intimidate bonus is just what it is.
In other words, this character has some bonus or other on Intimidate checks. How and why that bonus is what it is should be entirely up to you (or the player, if it’s a player character): the only limit is what the group will accept. And snakes for hair? As long as they aren’t biting people, as long as they don’t mean the goblin is actually a gorgon and has a petrifying gaze, then there’s no reason they have to be anything but a creative explanation for why your Intimidate bonus is what it is.