In order to do cash flow mapping I need zero rates for corporate bonds , where to find or how to find the o rates ?
This is a topic that can be covered by an entire book... Typically, you select some liquid instruments such as swaps, deposits, futures etc etc to bootstrap a yield curve, from which you can use it for discounting. The problem is not how to bootstrap, but what to bootstrap. It's a very complicated issue, you may have to consider cost of funding, liquidity, maturity etc.
Please read: A Practical Guide to Swap Curve Construction, you can find it on Google.