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.
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.
Robert Shiller at Yale has it monthly back to 1870 (obviously backfilling before the actual existence of SPX itself). This is the data used to compute the famous/infamous CAPE....
(Just add the dividends to price to give you TR).