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I use Photoflex softboxes. If you are searching for "speedlite specific" softboxes, you may be limiting yourself. There seems to be enough adapters out there now that most big-strobe softboxes can be adapted to small flashes.
I've got small, medium, and large Photoflex softboxes, and speedrings for both big strobes and speedlites. To use their softboxes with small flashes, search for their "basic speed ring" and "shoemount adapter." If you have big strobes, you can get speed rings for your strobes as well and use the same softboxes on both. They also make an adapter that'll let you use 2 speedlites with a softbox if you need more power.