Here you can download free data of the S&P 500 from 1789 until today (in csv format: Daily, Weekly, Monthly, etc.):

https://stooq.com/q/d/?s=%255Espx&c=0

(translation of the small window):

Zarzadzaj opcjami = Manage options
Zgadzam sie = I agree

write at the top left (in the box: Symbol np): ^SPX and then click on: Kwotuj

check that in the bottom left-hand corner the following is selected: Historical data

choose Interval: Daily, Weekly, Monthly, etc...

click at the bottom on: Download data in csv file

You can get the Indexes of the whole World, by writing in the box: Symbol np

DJI (Down Jones Industrial)

SPX (S&P 500)

Nasdaq

FTSE (United Kingdom)

DAX (Germany)

Nikkei (Japan)

Nifty (India)

IDX (Indonesia)

FTSE MIB (Italy)

etc...