Can you use Masser for Machtzis Hashekel?
Machtzis HaShekel may not be given from Ma'aser money. (Shaalos U'teshuvos Bais Dino Shel Shlomo Yoreh Deah Siman 1. See also Sefer Tzedaka U'Mishpat Perek 6 footnote 37. See also Mogen Avraham Siman 694:1 in the name of the Shela HaKadosh)
The Nitei Gavriel (Purim 26:8) says that men are not allowed to give Machatzit HaShekel from Ma'asar money, but women and children are allowed to (since they do not have an obligation to give the Machatzit Hashekel).
See also footnote 17, where the Nitei Gavriel says that according to the Magen Avraham one may bring the Machatzit Hashekel from Maaser (since according to the Magen Avraham, the Machatzit Hashekel is not a "Complete Obligation").