which one should I use and where can I get it?

Well what covering to use depends on what you are building. On my unlimited funfly planes i use clear ultrcote where i need the covering to be able to flex with the airframe and clear monokote where i need strength. The monoktoe clear is lighter than the ultracote but its tensile strength is higher so if it is used in a high flex area it will shatter....for example if you use monokote clear in the center section of a weakly built stick and boom aircraft then when you start banging out loop touch and go's the monokote will shatter in the center section from the wing flexing downward when touching the ground. On the other hand Micafilm is the lightest and strongest stuff i have ever used but some what more time consuming to install because you have to paint the glue onto your airframe before you can cover it.

I used Nelson's lite film... It was a little hard to apply, but the weight is really great!!! Lighter than anything else out there.

I have to agree Nelson's is very light and pretty flexable at that. But it is easy to apply just hard to get there lol. What I mean is when you do get the backing off of the covering you have to be very careful that it does not fold onto itself if it does you cant get it apart without it ripping very sticky stuff. But if you get it on where you want it just finger pressure and it will stay put till you can put a heating iron on it to spot it then stretch it as needed. I usually set the iron at 175 to 200 for this. Once you have the covering all tite and the circumfrance put down just use a heat gun to finish it just be careful because it is thin and you can burn through pretty quickly. But you will be able to tell as it starts to wrinkle up with the heat then the wrinkles just disapear then you move on. Used this covering on the wings and tail section of my WASP.