These might not match but the magnepan mmg w for 300 dollars is a dipole in your pricerange
You can also check out the Definitive Technology BP2X or BP1.2X Bipole surrounds. They can be placed either on a side wall or behind the listening position. The BP2X retail for $500/pr, but you should be able to get them between $400-$450, and the BP1.2X retail for $350/pr.
Energy also makes some nice Dipole surrounds, but I don't know the going prices for them.
I have decided on the Definitive Tech because I will be able to put them behind the listening position.