LOL go pirate. You must understand the magnitude of the risk by doing that. Installing unsigned and tamper-prone software on a device that's likely to hold your personal stuff. Dont come crying if suddenly the 'Sim 3' you downloaded off some forum starts making unsolicited calls to random numbers in Nigeria before MMS-ing half the content of your album to some strangers while deleting your SMS inbox, changing your GMail password and buying subscription to furry porn sites through your Google Checkout.

Just sayin' though.

Wanna ask ah, kalau dah install custom ROM kan, but decide to install back stock ROM, where can I get the stock ROM?

flash rom a bit hard la.. Any way I can just do this from home rather than finding a shop which would flash roms?

imho, the only best solution is to root.