There is no general online source of 1099-R forms. If you have an online account with the payer, you can probably access your 1099-R by signing in to your account. The exact procedure will be different for each particular payer. You might be able to import your 1099-R if the payer is a TurboTax import partner, but you might need a code or amount from the 1099-R in order to import it. The following FAQ has more details and instructions for importing.
https://ttlc.intuit.com/replies/3389470