Q: Where can I get a 45w magsafe 2 charger in Singapore?

If that is all you can find, note the following from Find the right power adapter and cord for your Mac notebook - Apple Support

Power adapters for Apple notebooks are available in 29W, 45W, 60W, and 85W varieties. You should use the appropriate wattage power adapter for your Apple notebook. You can use a compatible higher wattage adapter without issue, but it won't make your computer charge faster or operate differently. Lower wattage adapters will not provide enough power.

Another option is to use the following link to find an authorized reseller that may have the one you want: https://locate.apple.com/sg/en/sales/?pt=4&lat=1.2798008&lon=103.8372803