Use the search function to find previous posts on this topic.
US embassy's American Citizen Services (ACS) list shows 5 last I checked:
http://singapore.usembassy.gov.....apore.html
However, the embassy gives no comment nor assurance regarding their capabilities.
They only need to have some credentials and an office here.
Taxes For Expats (TFE) shows a SG office:
http://www.taxesforexpats.com/.....ional.html
They're not on the embassy's list yet.
Then, of course, there are the big name firms, e.g. PWC, E&Y, etc.
They're expensive, but this may be OK if you've got an employer paying the fee for you.
Another member of this forum suggested Artio Partners (I understand that they advertise on this forum):
http://www.artiopartners.com/
They're not located here in SG, but communication via e-mail is easy enough.