I'm looking for monthly data of the S&P 500 total return since 1956. I have looked into Bloomberg and Datastream, but it looks like there are data only from 1988.
My understanding is that there are multiple S&P500 total return indexes. Each has a different base year, 1936, 1970, 1988 and year to date. If you can't find the ticker for the different indexes on bloomberg or by asking support, you could try s&p themselves.
I was also able to find 1970 to present here(not sure on quality), http://centraltendencies.com/2011/03/sp-500-total-returns/
Edit: A source confirms my understanding,
There are four different total-return indices for the Standard & Poor's 500 Index: 1936, 1970, 1988, and year-to-date. Each one uses a different base period. The 1936 and 1970 total return indices were developed for historical use. Dividends are reinvested quarterly from 1936 through 1988 in the 1936 total-return index, and dividends are reinvested monthly from 1970 through 1988 in the 1970 total-return index. The 1988 total-return index is calculated based on daily reinvestment of dividends and uses January 1, 1988, as the base period. The year-to-date total-return index is also calculated assuming daily reinvestment of dividends; however, the base period is the last day of the prior year.