How to buy a SIM card in Malaysia
SIM cards are widely available in Malaysia from multiple carriers. Pre-paid SIM cards can be topped-up at ATMs, 7-11s, mall kiosks, etc. Malaysians must present their national identity cards. Foreigners must present their passports. They may also ask the address and post code of your local residence. Hotel addresses suffice.
It's hard to walk more than 10 meters in Malaysia without running into a mobile phone store or kiosk. They are more prolific than public telephones (and this isn't much of an exaggeration). If you still cannot find one, just ask anyone who looks older than 13 and you'll be on your way.
Mobile phone stores can be found on the 3rd level of Gurney Plaza. The most popular providers are Maxis, Digi, and Celcom.