I can't find the original text anywhere online.
Can someone point me in the right direction?
The Perseus Collection is a great online collection of ancient texts in their original languages (with a built in translation dictionary) as well as lots of different English translations. Here is the Republic.
There are also multiple scans of the text on archive.org. For example, here is Part I and Part II of Paul Shorey's 1935 publication, it has the original Greek as well as his English translation side by side.