The Mariners sell all their gear directly from their store in the stadium:

Safeco Field Team Store
1250 1st Avenue South
Seattle, WA 98134
Phone: (206) 346-4287
Hours: M-Sat 10:00-6:00, Sun 10:00-5:00
(hours vary during home games)
Store Manager: Craig Geffrey
Assistant Manager: Jackie Cram & Mary Beeman