tstenback brought up something that makes me think ...
A Regent filter, to the best of my knowledge, is just a Penguin, but without the BioWheel (this may not be technically true), right?.
If so, could you just purchase the correct size BioWheel and and clips, and slip it into the Regent? Or, is there a size/room issue?
Of course, the cost of just the BioWheel and clips, retail, would probably be about the same as a new Penguin or Aquaclear online.
And, personally, Stix, I don't have a Regent (I've got a Penguin 170, though), and if a Regent is a Penguin without the wheel, then I'd have to guess that it would probably be slightly better than a Whisper, which personally I think are the crappiest HOB filters I've ever seen. Certainly, I wouldn't expect your Regent to be any worse.
Lastly, if you can buy online, AquaClears aren't too expensive at all (at least in the US). Scanning the prices at Big Al's Online, I see....
Whisper 30 -- 14.69
Penguin 330 -- 20.49
AquaClear 300 -- 21.49
(Emperor 280 -- 29.99)