Problem solving is not completing calculations on a worksheet... it's putting real mathematics problems into the hands of students. It's not something that appears in most textbooks or websites, even though there are some exceptions.

Here are some resources that might get you started.

Mathalicious is a website with some free lessons and some you need to access via a subscription to the site - but definitely worth looking at to see examples of what are motivating lessons for students.

101 Questions is a website that is free but you need register first to participate and be able to access the graphics and questions submitted for the graphics. Add your questions for interesting graphics that inspire investigations and math questions! Great concept for a website resource!

dy/dan is Dan Meyer's blog where he provides resources for problem solving and challenges educators to make math "real" in their classrooms.

Here are some resources that might get you started.