From: Vancouver, B.C. Canada
Well, if you ask Mitsumi, they will tell you to buy Mitsumi brand media, which is actually made by Postech, and is about the crappiest stuff you'll ever find.
The fact is, Mitsumi created a CD Burner with a standard all their own, and no media manufacturer cares enough to make media just for them. In fact, most media manufacturers don't even care about 52x media, since 48x is by far outselling 52x CD-Rs. Of course, 52x is still being made in large quantities, but in a lot of cases is being sold as 48x since that is the most commonly requested speed.
So in short. There are no 54x CD-Rs, but you may be able to find something which will burn at 54x anyways. Good luck!
Canada; Leading the world in being just north of the U.S.A.