Where can I find crime scene photos online?

You can't. Those are the property of the police department that took them, and they are not released.

You can sometimes find photos taken by press photographers, especially from the 30s and 40s, when lurid photos were popular in crime magazines etc (black and white, of course). Those tend to be doctored, as when a photographer would spill water around a body, which when filmed correctly appear like blood pools.

Newspaper websites. Or just use a search engine like www.dogpile.com.

Why???