Currently there is only one CNG gas station in Jurong Island.
You'll need a special permission/pass to go there to top up yr
tanks. At the moment, most of the vehicles which uses CNG are
taxis, buses and vans. So i guess you can expect long queues.
Besides Jurong Island, there are also filling stations in Malaysia.
The list of the places where the stations are located are at the
website too.
http://www.cng.com.sg/howdoesitwork.php
But there are plans to open more filling stations in Singapore, one
of which will open next year at Mandai Road.
http://sg.news.yahoo.com/cna/20070712/tap-singapore-s-first-public-cng-station-231650b.html