I am looking for a source containing debt (government debt) to GDP data of different governments. Do you know of any reliable sources with data going back as far as possible (at least 1960s)?
Using the World Bank's International Debt Statistics you could calculate the ratio of PPG, bonds (AMT, current US\$) to GNI (current US\$):
PPG, bonds (AMT, current US\$): Public and publicly guaranteed debt from bonds that are either publicly issued or privately placed. Data are in current U.S. dollars.
GNI (current US\$): GNI (formerly GNP) is the sum of value added by all resident producers plus any product taxes (less subsidies) not included in the valuation of output plus net receipts of primary income (compensation of employees and property income) from abroad. Data are in current U.S. dollars.
These data go back to 1970's for some countries, but the data only covers 1970-2013. The OECD numbers go back to 2000 but are even easier to work with. You can link right to them. For some rich countries the data go back quite a bit farther (like G7 countries like the UK, France, and the USA). For a large panel of countries. I don't know of sources better than the World Bank and the OECD. You could check the IMF data page but all I found were links back to the World Bank data.