Marinaru

If the carbon paper became obsolete, you can use printers or photocopies.
If the printer is a jet one (ink), apply the paper on the balsa sheet and whet it with acetone. It will act same as a carbon paper, transfering the drawing from the paper to the wood (of course, the drawing have to be mirrored/flipped first, for special shapes...for ribs no need it). If the printer it's laser or a photocopy machine (toner), instead of acetone, use a hot (modelling or not) iron in order to transfer the drawing from the paper direct to the wood. I am doing this usualy to make a pair of templates and after, using these two templates I make a sandwich with balsa ribs and finishing all of them with a block sand paper. For an acurated job, use a soft pencil ( 2B-4B ) to make the edges of the templates, black. You will know when you have to stop because the black part will be erased by the sand paper.

I hand cut ribs 8 & 9. Now I know why laser cutting is so popular.

Garf wrote:

I understand that some sort of spray glue is used to attach the pattern to the balsa. Exactly what is it and where is it sold?

Nothing fancy, usualy it can be white glue diluted with watter. If you want something exotic, try 3M. I am using the 3M spray in order to fix the silkspan on a regular paper, to print diffrent things (laser or jet printing).