The main sources of problems I use are The Math Forum and The Math League.
The Math Forum problems of the week include both elementary school and middle school problems. These problems are very good for use in a 6th grade problem solving class because they include solutions submitted by students for past problems. 6th grade is the overlap year in The Math Forum problems between elementary school and middle school.
In the spring, the students at HUES can choose to participate in the math contests from The Math League. The Math League offers various contest problem books which cover problems and solutions for past math contests. Click here for a summary of how HUES and Hollis/Brookline Junior High School students placed on the contests over the last three years.
HUES 1999-2000 math choice (6th grade) sample problems: