I thought I already answered this...
You can find any item anywhere. There is no specific place to find a panacea shield, although you may have some luck farming Old Haven, as the Crimson Lance tend to drop shields often enough.
Places to look:
Chests - New Haven, Old Haven, Crimson Fastness
Skag Piles - Anywhere really
Enemy Drops - Old Haven, Eridian Promotory
Stores - Anywhere really, after playthrough 2 everything is lvl 48, otherwise the further store ahead in the game.