If you lose your EIN Confirmation Letter (CP 575), or forget to download it online, you can’t get another one. (Make sure you use a good mailing address when you apply for your EIN.)
However, you can request an EIN Verification Letter (147C), and you can use that instead. The IRS will issue an EIN Verification Letter for any existing business that has a Federal Tax ID Number.
Both forms are accepted by everyone (including banks and the state government).