For freestyle I like a large thick pad that covers the bottom and sides of the canoe from about 2 1/2 - 3 feet in front of the seat, back to where my toes rest and most of the way to the gunnels. This allows me to move about freely.
The best inexpensive pads are the foam interlocking squares, commonly known as "Toys R Us" pads. They can be cut easily with a utility knife to fit the boat. If cut to wedge snugly beneath the gunnels, they stay in place pretty well. Their downside is that they squeak when you move about on them, especially if the surface becomes damp. The squeaking can be eliminated by wearing long pants and socks.
Bell also makes several sizes of pads. Their large freestyle pad is rarely advertised, however last I knew it coould be ordered either through a dealer or direct.
Recentlt Cooke Custom Sewing began offering pads and from what I hear, they may be the best on the market. They are thick, have a comfortable cloth surface and a non skid back. Order from www.cookecustomsewing.com.
I've tried most of the available strap on type knee pads and found that when wet, they slip against the boat. I've tried them in composite boats, wood/canvas and Royalex. I've come close to hurting myself or dumping the boat several times while experimenting with them.
Finally, If the purpose is simply to have a comfortable, stable kneeling base while staying in the traditional 3 point position, the thick contoured foam knee pads offered by Voyager and others are excellant. They are comfortable and keep your knees spread in the proper position. I use them commonly in boats that I paddle on creeks and rivers as well as in my whitewater boats.
Dogpaddle Canoe Works