Where to find series of capital-output ratio for the US? While the capital-output ratio is widely used in Macroeconomics as a target in calibration, it is usually just set to be equal to 3, without specifying the data source nor the methodology used to compute it. After doing online research, I couldn't find any available data with the series of capital-output ratio.
Capital to output ratio can be calculated using the BLS productivity measures data. 
Capital to output ratio is simply K/Y.  Capital (K) is usually called either capital input or capital services and output which would be called real value added output.  

Which can be found here:  https://www.bls.gov/mfp/mprdload.htm   under 1987–2019 Major Sector Multifactor Productivity. 

https://www.bls.gov/mfp/special_requests/mfptable.xlsx

Or the World Bank group- Capital-Output ratio (NKY)
https://datacatalog.worldbank.org/dataset/capital-fundamentalism-economic-development-and-economic-growth/resource/babcd99a-16a3-4bef