There are commercial ones that can seal up to 95%. Anouther option is to put the magnet sheets over them to seal.
It's not always a great idea to seal them, especially if fed from the walls because unless the ducts are sealed tightly, the pressure will build and leak at connections in wall, thus wasting energy and possibly causing condensation problems inside wall with cool exteriors.

Thanks,zl700,
Could you tell me where I can find the registers you speak of?
I set up my system for zoning down the road,meaning there are4 t-stat zones wired, and separate ducts with baffles just off the supply plenum, so I can shut them off manually but have to climb into a mech-closet and it takes a while and is a p.i.a. I estimate the cost of 4-zones with electronic controllers and a central switching system at $2-3k installed. So I am saving up for that but wont happen for about a year. I did install a bypass return duct in anticipation of the eventuall zone system. In the meantime, I would like a convenient way to seal off about 4-5 of the 12 supply registers. I hear ya about possible condensate buildup in walls and subfloor, but want to try anyway, at least for 1 year till I can afford to zone it.

Any Hart & Cooley, Tuttle Bailey, Price or Titus Distributor,
You will pay through the nose. May wish to use tape or the magnetic sheets

Thanks, guys! I am going to try the magnetic sheets 1st. They had them at HomeDespot for $5/3-pak, so what the hell, right? If those dont do the trick, I am wondering if I can get a thick balloon material in there, blow it up with air compressor, and tie it off? Thought of that this morning all by myself! :)
Thanks again, for saving me dough and informing. As usual, love this site!

Because you are using an air tight seal on the surface of a device that is not air tight you will have air leaks and maybe that is not a problem for you but that is why we always suggest in line dampers instead of closing of air at the supply grille or diffuser.
You already have a bypass damper and so you will not have any negative impact on your system just make sure you adjust it so it opens when necessary. Your system was designed for a certain volume of air and if you do not give it that air you are bould to shorten the life of your unit.

Here is a link that might be useful: Air Conditioning and Heating Repair Made Easy

OK, now I know I am stupid. The magnet sheets would have worked fine except I have aluminum supply registers, and my "brilliant" ballon idea worked fine as well but deflated overnight. I guess I could try tape, or a solid grill (plate) replacement.
I am now thinking about what y'all have said about the dampers and maybe furthering my zone system just a bit, can be done.

Question: If I install the motors on the dampers I wish to open/close, is there a wireless controller available that will let me just close/open those dampers?
That way I can just open and close them at the dampers when i want, but not have an expensive computer controlling the situation. I am aware of the cfm needs of the furnace and coil and closing just 4 of the 12 supply registers will be OK, ie: still able to push 1200cfm over the coil.