This is liquor store where Dylan meets Gus, then follows him to his van in the alley to purchase the gun in the season 4 episode "Twenty Years Ago Today"
Liquor Store - Pat's Liquor Market, 6020 Kester Ave, Van Nuys CA
Gus' Van - Across Kester Ave, just north of Eugene's Body Works, Van Nuys CA
Note: See Windows Map link for view of both locations. Pat's Liquor Market is also the location from the famous Egg Exchange from season 2.