Your brokerage firm would not have Form 3921. Employers are required to send their employees Form 3921; however, now that you are retired, your former employer will likely not send you one. Your 1099-B reflects cost basis as zero because the brokerage firm did not know the price you paid to acquire the shares.
You do not want to include zero as your cost basis as you will be paying much more in tax than required. If you know when you purchased the shares, you can use a third party financial website, such as Yahoo Finance, to check the market price on the date you purchased your shares.
You might also want to review your prior W-2s. Box 14 is where many employers will enter additional forms of compensation such as restricted stock units or other forms of compensation related to equity options.
Lastly, maybe your former employer's benefits department might have the information you need. You might try contacting them.
@CFChase
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