The Post Bar at the Fullerton Hotel has some, but not the greatest selection. They were pricey but not double-retail or something crazy like that. I think there may be a store in the Paragon shopping complex on Orchard Road, where the SQ city ticket office is located. If you can't find anything, go into the Dunhill shop at Paragon as ask them for advice on the off chance they'd know. Given the general quality of the stores, Ngee Ann City would be a decent place to look.

The Post Bar is a good place to light one up, though.

If it were Bangkok, I could be of more assistance. Sorry.