I always struggle with sympathy cards as well.
I am not sure how the gallery works for you. I would go there, and under "search" type in sympathy. I always get a lot of great examples when I do that. Lovely as a Tree and the flowers from Never Ending Joy are two sets that come to mind. If you don't have a sentiment stamp that says something about "in sympathy" I am sure that "Thinking of you" also works.
Gallery at Splitcoaststampers
(This is a terrible post but shows one of my favorite styles for a sympathy card.)
I have also used a nice corner stamp - kind of a scroll like design with a similar geometric in the center, and the rest of the card fairly plain, with a sentiment stamped, maybe at the bottom I have not scanned this. I prefer the softer colors - greens, blues, lavenders.
But everyone has different ideas - look through the examples and i am sure you will be insprired.