Pretty well any softbox can be fitted to pretty well any flash-head: most of
them use a 'hub' for the box that attaches to a thing called a speedring,
and it's the speedring that is specific to the make of flash.  The 'hub' to
speedring connection is pretty much standardised, and you can also get
universal 'hub' fittings which will fit nearly any softbox, in case you have
one that you can't use a standard speedring with.

I have some redwing softboxes (Calumet own brand ones are identical) that
are cheap and cheerful and work well, and a couple of Bowens Wafers that are
better, but cost a lot more.  The Wafers are about the best there are, apart
from the Elinchrom lightbanks, like the Octa, which are a different league.
Elinchrom also make the Rotalux range which is very nice and easier to
manage than Wafers, and they also have a cheaper range under the Prolinca
brand.  Photoflex and Chimera are well thought of, and I know people who use
them, but I haven't used them myself.

(My studio flash is Elinchrom, and where I see catalogue lists of adapters
for Elinca, Bowens is also in the list, along with Bron and everyone else.)

Peter