Here is a site to download RPG Maker 2003 and RPG Maker 2003 RTP (there is no longer an official site).

The RPG Maker VX program is for creating games, editing games, and compressing them.

The RPG Maker 2003 RTP is needed for playing games. If you would like to play a game that is made with RPG Maker 2003, but don't necessarily want the program you can download the RTP and play it, or hope that their game includes RTP.