What's the typical practice in state court when there's nothing on
point at the state appellate level? Do folks somehow find and cite
state trial decisions? Rely exclusively on federal decisions?
Federal decisions even from federal trial courts, then, secondary authorities (especially the Restatements, official commentaries to Uniform Laws, and legal treaties) are the next most common sources.
State trial court decisions are sometimes cited (usually with a copy of the decision attached) but much more rarely since they are harder to find, usually to give comfort to a judge that others have made the same decision urged. Typically these are offered by law firms for litigants that regularly litigate these issues, such as a prosecutor or AG's office, a public defender's office, or a firm with a significant litigation practice in the area in question with a ruling they personally litigated.
Unpublished state and federal appellate court decisions are often cited in the same vein.
Appellate courts often have strict rules prohibiting the citation of unpublished opinions not involving the same parties, but state trial courts are often more flexible.
Alternatively, someone will often take an unpublished decision at the appellate or trial court level and simply make the argument made in that decision citing the same authorities and using the same reasoning without citing to the source.
Beyond that, lawyers look for analogies at high levels of generality, for logical arguments, for non-U.S. law, under historical rulings that are no obsolete for some reason, and for policy arguments.