A wand from Ollivander's will get you through your school days. But if you have a certain talent or at least particular inclinations, sooner or later you must find Alimah's little store. It lies just shy of the respectable end of Diagon Alley, through a certain side door. Track the scent of mingling incense, attar of roses, and oudh. Listen for the strains of a distant melody that you have never heard before. The door is always open, day or night. Turn the handle and go in.
Don't mind the smoke, or the drapes, or the two dead birds. Ask for Aria, or Sangeet. Call them. They will come. They don't speak much, these dark haired girls, but it is no matter. Show them what you've brought. They will know what you need. Mind your step when they beckon you upwards. The staircase is rickety and old. They repair it only enough to keep it creaking in tune. When they show you the room, with its velvet hangings and cushions of brocade, and shelves and shelves of what you need and what you don't and things you don't even recognize, breathe. The incense is lighter here, but the music will be at once more noticeable and harder to hear. Remember what you came for. The girls will show you what you need.
Pernambuco is always good, if they offer it, springy but strong. Even the muggles appreciate it, you may have heard. And unicorn hair strung across, though the muggles only have horse. Take hold. Give it a try. Set it to your instrument and play. The girls will know before you do if the tone is right. Maybe it's poui wood and veela hair that will respond to your touch, if you're meant to make your listeners dance despite themselves. Banyan tree and centaur's tail to play with stately, ancient grace. A viola's not played well if not with mermaid hair, though sometimes the spark comes with banshee instead. You'll want baobab if you play bass.
There was one cellist who came away with a bow of ipê wood and giant spider silk. Her next concerto made the furniture sprout and bloom. One violinist brought every hall to violent riots with a bow of jubokko and harpy hair. If you're not ready yet to run that risk, to lay evident to your listeners your very heart and soul, any bow will do, if haired willingly by the wood's own dryad. But if you're serious about this, keep searching.
Sometimes it's happened, once or twice, that neither Sangeet nor Aria could find what's right. Sometimes, if you're lucky. Then you'll hear silk-slippered feet coming down the attic stairs. It's said no one has ever seen the Grand Dame Alimah, and in a way it's true. She is old, and she is large, and walks slowly, and she is veiled. Some think the dark eyes set in the strip of brown skin aren't human, that the niqaab hides an aging siren, or a djinn. The music follows her wherever she goes. Don't bother trying to catch the melody. You won't remember it no matter how hard you try.
She will look at you, just once. She will not say a word. Then she will reach into the shelves and hand you your bow. Accept it, whatever it is, be it dark oak or Yggdrasil. You came for this, a different kind of magic. Take it home. You don't have to play well right away. Just play.