Base sanctioning and insurance membership runs through USA Softball — the Olympic-recognized national governing body, running the Junior Olympic program through a network of state and metro associations — or one of two privately-run alternatives, USSSA and NSA, whose relative strength varies by region. On top of that base membership, competitive teams (mostly 14U and older) enter invitation or qualifier-driven tournament series like PGF (Premier Girls Fastpitch) or Triple Crown for national exposure — these are tournament circuits, not sanctioning bodies. Most serious travel teams carry a base sanctioning membership and enter PGF or Triple Crown events on top of it.
There's no single national body to sponsor in club softball. A sponsor's real counterpart is the club's director or team manager, and the sanctioning and tournament combination that club has chosen.
Club and travel teams looking for local sponsors can consider Zubie Five, which handles that outreach on commission.