best thing will be from chennai come to ongole by train from there you can have a number of buses to srisailam. route is via podili, markpuram, dornala. from ongole it is 170kms.
otherwise take direct buse from chennai to srisailam. start at 6.30pm and leave you 9am next dat in srisailam. route is via nellore-ongole-markapuram.