There's another company that makes a BK style bar, but it is very expensive, like only around $50 less than the BK bar:
http://store.gospecracing.com/gsrautosportrollbarextension.aspx
(The page seems to be down right now but this is the bar)
Best bet is a used BK Bar, I have bought 2, one for $350 and one for $400 including the BK pads (the $350 one had traditional rollbar style padding). Some folks with track only boxsters run them with no pad and only when wearing a helmet.
I think there may be one for sale right now on the board here.
I don't know of any other bolt on options for the Boxster except for a bolt in roll cage.
Steve