My version doesn't look exactly like Helga's, but that's what makes it so wearable for girls everywhere if you don't like people to pick you out of a crowd thinking you're wearing a costume such as this girl:
Without further ado, here is my version!
The dress I plucked from Forever21.com, the locket is from Etsy.com, and the shoes are from Keds.com. I guess one could find a plain t-shirt like this anywhere, but I found the pic from Google. Finally, the ribbon can be found at most craft stores.
This was a really fun project, I may do outfits for more characters from Hey Arnold! in the future, but until then, have an awesome today (because I don't know what day you'll be reading this)!