You can physically put far wider tires on the bike than will fit properly onto the rims. I've done it and paid the price with poor handling as the tires wear off the top of the crown (fairly quickly). There really isn't any need for wider tires, as it turns out. Good, proper size tires will still let you drag the pegs in canyons. What's the real need for wider tires? The extra weight sure doesn't help.
Though most shops don't stock them in house, the standard sizes are available and can be ordered. They would rather sell what they have and will do their best to talk you into "modern" tires. I recommend you resist. Few sales men have any riding experience with the Cb550. The bike suspension and geometry was tuned for original size tires, not the modern ones with lower profiles and different tread shapes. The right tires and you can steer the bike by subtle leaning instead of wrestling with the bars to make it go where you want.
I use the Metzler tires; 3.25 19 front and 4.00 18 rear. They are the right profile, shape, and stay properly crowned with wear, assuming you do lean it over occasionally. The old inch sizes still have modern rubber compounds on them. An, I think that is more than enough improvement fro this bike.
Cheers,
