A source for what?
Personally, I think it depends on the applique and how large the pieces are. If they cover a large area, I like to quilt details into them, such as veins into leaves, bubbles on fish, etc. If the appliques are small, a lot of times I'll just echo quilt them. Smaller appliques also look really good if you quilt densely around them, they pop out from the quilt a bit.