The Void Star can be found on the first floor of the Clothing Store and purchased for 55 Bucks. Purchasing this hat will allow you to wear this hat only in Adopt Me!, unless you buy this item from the Roblox Catalog.
The description of this item on the Roblox Catalog reads
"Void Star to zer0, Foolishly dereferenced. No call stack. All g0ne."