I agree with the people saying this would be an excellent first sewing project buuuuut that would require buying a sewing machine and taking time to learn a whole new skill which is significant time and money.

Instead you could get any white cape with a hood and make some easy alterations to it to match what you're going for.

If you're really set on buying one I think this could be a good option:

https://www.amazon.com/Futurino-Gothic-Hooded-Poncho-Outwear/dp/B09G5PG6D5/ref=asc_df_B01MFAO5MM/?tag=&linkCode=df0&hvadid=312680966058&hvpos=&hvnetw=g&hvrand=15605874603313191580&hvpone=&hvptwo=&hvqmt=&hvdev=m&hvdvcmdl=&hvlocint=&hvlocphy=9008479&hvtargid=pla-572499770821&psc=1&region_id=972485&ref=&adgrpid=62541972632&th=1