Is there a good data source that aggregates the MPCs of various countries?
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You may need to do some math but the OECD database does have the data that you need to construct MPC by country. As a reminder MPC can be calculated as the change in consumption (ΔC) divided by the change in income (ΔY).
For consumption the OECD data series you would need is "Household spending" https://data.oecd.org/hha/household-spending.htm#indicator-chart.
For income the OECD data series is called "Household disposable income" https://data.oecd.org/hha/household-disposable-income.htm#indicator-chart
Download the available data and calculate the growth rates of each measure then solve for MPC and you should have it. If you have trouble with the data download contact the OECD for assistance.