Andrea makes several sets of dry transfer heraldic charges. These are a variety of charges, scales and colors. They are also broken up into regional influences as well. French, English, Italian, German, secular and religious.
If you want to try something by hand, start off easy. Crosses, simple field divisions, rondels (circles), Mullets (stars), a general book likethe Fox-Davies Heraldry will give you plenty of simple heraldic ideas to warm up and as you become more comfortable you can try an ever increasing difficulty of things to paint.
A word of advice here too. And this is my personal opinion.
We tend to look at things with a modern artistic aesthetic when it comes to medieval art and representation. Look at the medieval manuscripts that illustrate heraldry and you will see something that looks rather childish in execution. Manuscripts like the accounts by Joinville, Froissart, De Charny, Gerald of Wales, Mathew Paris, and books of hours and bibles like the Macejowski, Bedford Hours, Luttrell Psalter, Manessa Liederhandshrift, etc, illuminations from the 10th c. to the 13th-14th centuries, all have what we would consider today to be very crude depictions of heraldic charges like beasts, objects, people, symbols, etc. Not well executed, Frazetta, Hildebrandt, Waterhouse, Soriyama, Howe, Embleton, fine illustration. While individual style can vary and there are examples of very well proportioned and illustrated subjects, it wasn't all like that. Even in the modern era, a quick glance at aircraft nose art will give you a selection of artwork that looks like it came off some grade school kids notebook doodlings running the gambit to an Olivia or Vargas pin-up. And several very bad copies in between of the latter.
Modern expectations are a result of the machine age, the appreciation of a photo realistic illustration and or manipulation and a wealth of very talented artists that have a level of exposure to an audience un-parralleled. Yes, some if not most judges will be impressed by a modern illustration but its ironic that they miss the point all together when they look for period authenticity in the execution of the figure and its clothing and applaud some of the most blatant anachronisms on the piece.
In short. When doing medieval representations of heraldry, don't look at them with a modern eye, look at them with a medieval eye. Any book with illustrations of heraldry showing reproductions of those blazons will prove my point and allow you to relaxe a bit regarding your painting. Be wary of Victorian era publishings (they tended to "fill in the blanks" a bit too much) and look for medieval artwork.
When you are ready to paint your own, take a piece of card stock and paint a base coat on it. Take your shield or a stencil of the shield and draw an outline of it on the painted stock. Now lightly using a pencil draw your charge and then go over it with your base coat of paint. Build up coats and detail over several applications. This way you get the benefit of "practicing" before you tackle the actual figure itself. One of the things you'll find is that artists break these art pieces down to a series of simple brush strokes. A dragon may start out as an "S" shape and everything else is built up from that.
Good luck and have some fun with it.