You can still find it in MIMS sg , see link
http://www.mims.com/Singapore/drug/info/Bismag/?q=Bismag
I have not seen BISMAG at the medical halls or pharmacists for a very long time, not that I use that medicine.There a few common medicine we use to see on the shelves have disappeared.
One example, I do not use Agarol ( laxative liquid, pink in color), I have seen this in shops since I was very small. I rarely gets constipation that needed laxatives, once years back I did need it. I went to the shops & was told that Agarol was discontinued here. Why, ask Min of Health...but you can find that every where in you know where!!
There is another for de-worming a medicine brand name - Conbantrin, that is also discontinued here, see link
http://www.combantrin.com.au/ I used to take this once a year to clear the worms in the intestines. Ask Min of Health why.....
They want fees from the manufacturers, that is why they discontinued selling these medicines in SINgapore, I am sure that includes BISMAG too.