Getting the Last Accessory in Littlest Pet Shop (DS)
Finding the Final Accessory in Littlest Pet Shop DS
I don't have the exact unlock condition for the last accessory—it varies by specific Littlest Pet Shop DS title (there are several: LPS, LPS: Friends, LPS: Beach, etc.).
Where to Find the Answer
GameFAQs: Search "Littlest Pet Shop DS FAQ" or "accessories guide."
YouTube: Search "Littlest Pet Shop DS all accessories" or "last accessory location."