The three clusters
The Arcade (Raffles Place) for CBD-sharp rates, Lucky Plaza (Orchard) for convenience, Mustafa Centre (Little India) for late-night and exotic currencies.
Read the board
Boards show we-buy / we-sell per currency. The spread between them is the changer's margin — tighter is better. For amounts over a few thousand, ask for the dealer rate.
Stay legit
Every legal counter displays a MAS money-changing licence. Never change with touts outside the licensed counters.