When you begin to draw your own heraldic design, it helps if you can draw. I can't, so I relied on the Internet to find clip art and even heraldic software. By far the best of the lot was Blazons! 2000. You can try it for free, and registration is only $45. One big limitation of the program is that it only comes with a set number of images. If the stag is leaping in the wrong direction, you can seem to be out of luck. Fortunately, I discovered that the little files for the stags and stars and other charges are in the GIF format, even though they have a different file extension. You can open the stag in Corel Photo Paint, flip it, and save it. Presto! A stag counterspringing. You can add custom artwork the same way.
The author writes on his Web site that the program is "NOT terribly useful for SCA heraldry. (Most are much too complex.)" That's not the fault of the software, but the fault of the artists. Keep your design simple, and this program will help you translate the heraldic jargon known as the "blazon" into pictures.
There are many other sources for heraldic graphics and software. I found some nice designs for my kids with this freeware program called "Shields, Knights and Heraldry." Some of these images scale up better than the ones in Blazons! 2000. This educational Web site also has an on-line game about Heraldry and medieval life.
As for other sources, here is a list of heraldic software on the Web. I won't try to duplicate it.