Go to the problemset tab and on the right you should see a rating filter. div2a problems are probably around 800-1100. The good thing about this is 1) you can be as specific as you want regarding difficulty and 2) this will pull up all problems that are at the difficulty of div2a for example, and not just div2a problems (thus providing more problems to practice with) and 3) this won't include unusually hard div2a problems that would normally fall in the div2b range.

EDIT: The tab is labelled "filter problems" and the lower/upper bound for rating can be entered in the two boxes labeled with "difficulty:".