You could try the IMF which publishes public sector spend data in their World Economic Outlook Database. This is available free-of-charge at the following link:
The potential proxies for public sector budgeting depend on the government structure and administrative organization in each country. However, in many countries, there is a ministry responsible for preparing and implementing the government budget, and this ministry includes a range of different agents and departments. Key potential agents could include: (Ministry of Finance, Ministry of Planning or Development, government sector ministries, independent regulatory bodies, legislator)
Please note that these structures and nomenclature can vary from country to country. Therefore, you should always research the exact details about the structure and organization of the public sector budget in the country you are interested in.