Beta sites don't get migration paths - usually. I believe there are a couple of odd exceptions but, in general, assume that beta sites just don't have them - in or out.
Even when a site is graduated, most sites don't have any/many default migration destinations as we restrict the list to five and we expect to see a history of frequent migration to the site before we'll activate it.
Moderators on all sites have the ability to migrate posts from the sites they moderate to any site on the network, though. So, if a question is completely off topic for Sustainable Living, flag it with a moderator attention flag and ask that the question be migrated.
Please note, however, that being off topic isn't necessarily a reason to migrate. If the question is low quality, it should be closed even if it might be on topic elsewhere. If a question is on topic here but a "better fit" elsewhere, then we assume the user chose to post it here, so we discourage migration unless the OP requests it.