to add to what soumitra has said, u can also opt for it at the time of booking itself. many a times it is not supplied to passengers demanding for it onboard. so if u r sure, opt for it while booking at prs/irctc.
please wait...Translate to Hindisoumitra ne kya kaha tha, usme aur bhi cheezein shamil kar sakte ho, booking ke samay hi iska chunav kar sakte ho. bahut baar log onboard maangne par isko nahi diya jata hai. toh agar tumhe yakeen hai, toh prs/irctc par booking karte samay hi iska chunav karo.