You can try this stall outside Orchard Emerald, opposite John Little at Specialists Centre. Its a small stall selling perfumes and they have a huge range. Their brand new testers are especially cheap and I normally buy them for personal use. But if you're getting the actual product itself, they sell it cheaper than most stores too.
I bought my Giorgio Armani Acqua di Gio (tester) 100ml for only SG$58! And also a gift for a friend, Kenzo flower (actual, NOT tester) 30ml for only SG$40.