if you google the words boston, homeless, services for the homeless in boston, boston homeless shelters, boston department of human services, you will find that boston actually has a lot of services/ programs and shelters for the homeless.

you can walk into any department of social services in boston and they will point you in the right direction.

since you are on SSI you will also need to report your living arrangement change as in letting them know you are homeless, they will also have listings of help/ and resources for the homeless.