Before the hobbyist availability of Sheet Inkjet and Laser printable waterslide decal papers. Dry transfer lettering was and still is to a much lesser extent used on custom Scale models. but I have to say if you have access to a computer and standard Inkjet printer creating your own Waterslide decals is by far quicker and in the long run cheaper then dry transfer lettering.
Over the years I've used all kinds of On-line sources for sheet size (8.5 x 11) clear and white decal papers. but I have to say the Very best quality and shipping has always come from Micro-Mark. Their Decal papers always arrive damage free with no dog ears or folded sheet damage I've experienced with most of the on-line suppliers.
www.micromark.com has my highest recommendation for both laser and inkjet printable water slide decal papers.
Do Note! all inkjet inks must be clear coated before cutting apart and must leave about 1/16" of border around the print to prevent water ruining the decal. I print ALL my own decals now using this paper and Krylon #1306 Workable fixatif (1st sealing coat) followed by Krylon #1305 UV Resistant Clear.
Please note that Krylon #1301 Crystal Clear and most other Rattle can Clears contain 0% UV inhibitors meaning they Yellow very quickly when exposed to sunlight or any UV source. Choose a UV resistant clear final coating for your printed decals.
Another good product if you can find them locally is Micro-Scale clear liquid decal film and Micro-Scale setting solution. I've not used their decal paper but if it is as good as their liquid products they should be very good.
Back to Dry Transfer lettering. As Dan mentioned they can be applied directly to any painted surface curved or flat by using a mandrel inside the tube to support the surface. use a circular motion starting from the center of the letter with the burnishing tool, rounded end dowel or what have you. I always apply some sort of protective wax or polymer coating rather then Clear coats. Clear coats ALL become brittle with a little age chipping and flaking with Handling and/or landings which also take a bit of the Transfer or decal with it. NU-Finish Polymer or Pledge w/ Future (for Tile) are much better "Protective Coatings".