Winston: Tops are pretty easy to find on the internet. All you really need to know is the width and the height you want. Then decide if you want two, three or four bows. I think three bows is typical for boats our size.
Sunbrella is good material and a little more expensive then some of the cheap vinyls out there. I found that dark navy matches the Grady color.
Look at "Chum Along's" photo. See the way that the top material curves down the frame a few inches? This helps hold the top in place - keeps it from sliding left or right, - as well as giving a bit more sun protection. Cheap tops may not have this.
Also, look at how the straps are held in place on the frame. Cheaper tops just depend on the top material itself holding the straps in place. These tend to slide around. A better top will have a way of mounting the strap to the frame, ususally a screw and finishing washer.
Aluminum is good frame material. Stainless steel is better, but is also much more expensive. But not all aluminimums frames are the same. Compare the wall thickness. Good tops will also have long inserts where two pieces of the frame come together to add strength to the joint.
After you do all that, even a good top will have a tendency to sway from side-to-side a bit. The shorter the top the less it sways. I added a couple of U-bolts to my windshield frame and run a short bungee from the U-bolt, around the frame and back to the U-bolt. Not much pressure, just snug enough to keep the top from swaying side-to-side. This has worked really well and I have almost no movement of the top when running.
Hope this helps.
Rob