S-grade can be obtained by anyone, they're items you craft the same way you'd craft B/A-grade.
There's some high lvl quests that give S-grade recipes/parts and some raidbosses drop S-grade items I believe. Also S-grade duals you can make by combining 2 a-grade swords.
And no, S-grade do not disappear at the end of the month.
Heroes get Hero weapons after they've won the olympiad. They choose a weapon they want to have, and then they have that one for a month. After the month has passed and new olympiad starts, they lose the weapon and hero status.