If cost basis on 1099-B is not reported because I moved my ESPP shares to another broker, then how can I calculate cost basis for shares sold acquired in diff times?

I have ESPP shares from my ex-employer, which I got before 2012. In 2017, I moved those ESPP shares to another broker and in 2018 sold few shares through this new broker. 1099-B sent by new broker does not list cost-basis as they don't have acquired times and FMV of the shares to report cost basis. In this case how to I calculate cost basis?
Thanks in advance.