Either from Institutional funds or from fellowships offered by the DBT. Most Institutions also offer nominal-cost housing to post docs. Grant funds may be used to pay post doc stipends (if you have lots more post doc applicants than your Institution's fellowships allow! but sadly, see the next FAQ. There are no additional benefits/overhead costs, so typically the only amount that is to be budgeted for is the stipend and HRA (house rent allowance). Stipends are under Rs. 20,000 per month, and HRA is an additional 30%. So the total costs of a postdoc salary on a grant are approx Rs. 3 lakhs per year.
CSIR now has a Nehru fellowship that offers 35,000/month (only for CSIR labs). The Wellcome-DBT India Alliance early career fellowships are very high-paying- check out their website.