Spells in Journeyman are limited by ones ENDurance, so naturally mages look for a way to boost their END. Here are two ways. There are probably more.
Chewing Coca Leaves: In areas where coca grows naturally, coca leaves are chewed by some people for relief from bone-numbing labor. The result is +2 END, -1 INT and -1 WILL. PRICE: 100 p. per dose (if imported). Each does lasts 1 week.
Ring of Endurance: This rings grants +3 END to the wearer. PRICE: 100,000 p.