If SSI was your only income your were not required to file a tax return. Hopefully the IRS will process your stimulus payment using the Social Security files for your SSI. You should receive the payment in the same way you get the SSI payments.
You can track your stimulus using this portal: Get My Payment | Internal Revenue Service
If you have a dependent under age 17 at the end of 2019 that you need to register, you have until May 5 to do that on this portal: https://www.irs.gov/coronavirus/non-filers-enter-payment-info-here