Since you have moved from one broker to another, you have two ways to get the cost basis --(a) your own records such as statements from your first broker or (b) your old employer may have data on the acquisition cost of the shares at the time of purchase i.e. historical data. If the employer was a large employer whose stocks are publicly traded ( or even OTC ones ), there are financial sites like Yahoo that have historical prices available. Note that when you file your return, you do not have to provide proof of Cost Basis. However, you must have documentation backing up your cost basis , in case of an audit.
Hope this helps