You may want to try KJ at Bukit Timah 6th ave (the one shifted from Tiong Bahru).
One thing, you've to take note is, slide development will take around 3 days to process as they dun have any development machine at their retail store anymore. (from this month onwards), they will send the films to their HQ for development.
The price is a bit steep though around $5 for unmounted slide.
i used to use KJ extensively but stopped after realising (from slide scanning) that my slides were sometimes scratched! switched to RGB which was cheaper as well. you'll get a discount on E6 if you're a SAFRA member.