Polars for multi element foils?
Your question is too general. It would be more helpful if you specify the flight conditions and the types of aircraft that you are interested in. For instance, give us a range for the Reynolds number. 

However, I have a fast answer for both high and low Reynolds. For high Reynolds ($Re \geq 10^6$), NLR-7301 (main and flap) and MDA (slat, main and flap) are airfoils for which you can find published experimental and numerical results (NASA, AGARD, etc.). For low Reynolds numbers ($Re &lt; 10^6$), there are really not many experimental results that I know of. In fact, I was once lucky enough to find an airfoil catalog of F.X. Wortmann (printed format and in German) that had aerodynamic results for some fowler flap airfoils. Also, during my diploma thesis, I worked on an aerodynamic method for the fast analysis of multielement low Reynolds airfoils (based on XFOIL) and thus I used my method to analyze certain multielement low Reynolds number airfoils that I designed (starting from the S1223 airfoil of Selig). The document can be found here but it is written in Greek. Nevertheless, you can check the figures (NLR-7301 is also included in my study and you may trace the relevant references for further info).