I don't think we have this for sale in Singapore. You "might" probably find some in tubifex worm bags sold at the LFS. I say might because there is always these peculiar worms in the tubifex that are usually much bigger and much darker than the tubifex. To identify them you have to do a test. If you touch them, they curl and swim away, those are blackworms. If not, they are tubifex worms or similar worms allied to tubifex worms. You can probably try the shops that deal in frozen fish foods, but as far as I know, Hikari at least, does not produce nor sell frozen blackworms.
Read here for info on blackworms:
You can probably get a live culture from the US or overseas via eBay or Aquabid, but whether they survive the transit time in the post from the seller to you is another issue altogether. Whether the Customs here allows such a culture to enter the country willingly is also a doubt.