Although it might be pretty easy to find someone willing to make you a robe online, especially with all the Harry Potter costumers out there, robes are relatively simple to do, and getting in the habit of making your own costumes is more economical and creatively rewarding in the end.
That said, classic mage robes are pretty easy because there are a lot of patterns readily available to make them. If I were you, I'd start out by finding a cloak/robe pattern either online or at any store like Walmart or JoAnn's. If you're unsure, find some scrap material to make a "first draft" of your costume (an old bedsheet works great for this kind of thing.) Once you feel confidant, you can use your final white fabric. The triangles can then be either be sewn on using red fabric or painted on with fabric paint.
The great thing about FF1 costumes is that they leave a lot up for interpretation - my main advice is have fun with it - do whatever you want! You could go fcor the classic robe or make the robe shorter, use a cloak and a white dress, etc. You have plenty of options with those 8-bit sprites.