Where can I find historical options charts data specially for stocks options for backtesting?
Only current month, next month and far next month data and charts are available on Kite.
Where can I find expired months charts?
Yes OPSTRA is paid. But if you pay for just one month you can test and may be prove your well thought out strategy. Then live trade and make money with a working strategy.
NSE provide paid historical data so far I know. But you can switch to OPSTRA for backtesting as you don’t get particular options backtesting feature from NSE but on the other side OPSTRA provide backtesting upto year 2019 options.