However, the best source is the Empolyment NEWS, December edition of every year. It is cost effective because it can be had for mere Rs. 5/. Secondly many publishers print booklets containing syllabus only which could be bought for Rs. 50/- to Rs. 60/-. Similarly, the question papers of last years are readily available in the market. You must posses them. They can help in making your preparation more effective.
The next best option is UPSC site. Look into the notifications.