Where to get Weekly Shonen Jump in Singapore?

If you mean a physical copy of the magazine- you can't. It's only available to buy in Japan. However, they have an app where you can buy the manga digitally. You can subscribe to it as well. I don't know if I should mention it but in all issues they sell (On the app) they say you won't get any Yu-Gi-Oh! cards if you subscribe. It's just a heads up as in Japan, subscribers get Yu-Gi-Oh! cards every issue.

If you mean the manga volumes published by Shueisha (The company that Run Jump), you should be able to get them in any good bookstore.