Most of the niche perfume-oil houses like BPAL (Possets, ZOMGSmells!, Arcana, Villainess, Alkemia, etc.) will sell imp-size sample vials, and some of the niche conventional perfumers like CB I Hate Perfume sell sample sizes too. Lucky Scent sells sample sizes of most of the niche perfume lines they stock (dunno if they're decants or official sizes), and The Perfumed Court is an *amazing* source for decants of everything from imports to discontinued/vintage fragrances; they also have some pretty amazing themed samplers if you want to get testers to explore a particular style or note: http://theperfumedcourt.com/Categories/Samplers--Sample-Packs.aspx

For manufacturer's samples or miniatures from the more conventional department-store lines, Fragrance.net sometimes has them in stock, and they can frequently be found on eBay too.