Amazon offers Prime members the Sony PlayStation Vita Protective Film 2-Pack for $1.98 with free shipping. (Non-members can spend $35 or more to qualify for free shipping; otherwise, it adds $4.98.) That's the lowest total price we could find by $3 . This official Sony product protects against scratches, dirt, and dust.
Update: Note that this item is temporarily out of stock, but can still be ordered at this price for delivery when available.