hCG obviously directly acts on the testes, but it's a drug and not a supplement.
D-Aspartic Acid has been shown to increase StAR protein levels in the tests, but also has effects in the pituitary.
You can also look into Coleus Forskohlii/Icariin which increase cAMP and may also increase StAR (You can read about it under Technical Info at Invalid Link Removed )