Hoodie lovers unite! You can never go wrong with an awesome hoodie. Personally, I'd suggest owning one for every occasion. This particular hoodie -- which is unisex, so you only need to worry about the size -- is a 60/40 cotton-polyester mix in white. It features some cool looking art of Asriel in all his forms -- as well as his rather sinister-looking Flowey-form.
It sums up Asriel's story line in a pretty attractive way.
This hoodie is being sold by seller yilv45 who has a 100% positive feedback rating with 528 ratings on eBay. That said, they are shipping from China, so the copyright-related legality of some of their items might be a bit questionable.