Yeah, the Kama Bay is way too expensive and the fan that comes with barely moves the air. The Aerocool Masstige series of cases (Masstige, CoolView, etc. - their stacker type cases) come with a 5" bay fan holder - nothing to write home about but it should be cheap to get as a replacement part, but you'll probably have to deal (social engineering skilz will probably be useful) with the US importer to get one (I've not seen one at any of the reseller sites that sell those cases - I have a Cool View, so I've spent some time looking)... It should fit most cases. Another option is to glue three of your bay filler plates together and then rig them as a fan holder. Yet another is to get an HDD cage like the Tt iCage or the Cooler Master Stacker or the Silverstone CFP52B cage (I call it the Kublai Kage as it was designed for the SST Kublai series of cases and will need some modification to hold the fan housing in place on most other cases) and use just for the fan mount. Unfortunately the CM one can't deal with a 38mm thick fan and keep its dress bezel (neither can the Kublai Kage), while the Tt one can as long as you don't mind the bare fan and front of the cage showing - ditto with the C-M cage. Those are reasonably priced, but the fans that come with them aren't all that strong either - at least you get two functions for the money.
.bh.