Here's one place where you can compare prices, all on polk surround speakers:
http://shopping.yahoo.com/s:Surround%20Speakers:4168-Brand=Polk%20Audio
Another place:
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&DEPA=0&Order=BESTMATCH&Description=polk+audio+surround+speakers&x=0&y=0