Smaller speakers from the KG series are a good bet, as already posted. I used the KG 2.2s at one time and got a great surround field that blended quite well with my Forte fronts. So, any from the KG 1.2, 1.5, 2.2, 2.5 series should do well and are not terribly hard to find. Perhaps others will comment on their experience with other choices, such as the KG-1, KG-2, or other lines. A great deal of rear channel material from movies is ambient, incidental sound, so an exact timbre match is less critical than it is for the front three speakers (exceptions being occasional front-to-back or back-to-front jet flyovers or some such unique/rare effects).
Perhaps as critical for the rears is your ability to position them optimally. Most people will, by room necessity, have the rears closer to their listening position, compared to the fronts. Being able to aim the rears toward your head, and get their output level balanced with the fronts, will make a big difference in acheiving a seamless blending of the multiple speakers. This might argue in favor of smaller speakers on stands, unless you have a large cabinet or shelves behind your listening position.