Best 9 speed cassette for fast & flat
My guess is that if you have 12-28, you might not have a Shimano 9 spd hub. Generally Shimano road clusters top out at 27t. That sounds more campy-like to me. That's the first thing to figure out. Shimano and SRAM clusters are interchangable, Campy are not. 

If you have a shimano hub, it's very easy to build 'custom' cassettes with Shimano 9spd parts. It's just not cheap. If you've got the tools to remove the cassette, you can get an 14-25 

http://www.chainreactioncycles.com/us/en/shimano-tiagra-hg50-9-speed-road-cassette/rp-prod387

and mix and match with your current cassette to get the gears you want. It may be possible to do the same kind of mix/match with campy 9spd cogs, it's just more expensive. You can get 14-28 campy cassettes. 

http://www.universalcycles.com/shopping/product_details.php?id=5047&amp;category=40

If you can find them, junior racing cassettes are a good solution. Junior racers have gear limitations so they tend to have clustered but lower gears. Currently they only seem to be available for 10 speed cassettes, but you can get 16-27
cassettes. 

If you poke around enough on the web, you can find small companies like IRD that will make all kinds of custom cassettes. They aren't cheap though.

http://interlocracing.com/cassettes_steel.html