For adding sign language representations (think sign language "captions") to digital media, there are the Sign Smith Studio and ASL Animations products from Vcom3D, for adding avatars and animated graphics, respectively.
Additional free resources online:
- ASL Clip art page of the alphabet
- Gold Clip Art's Letters and Numbers
- Alpha Info Products offers some free animated clip art of signed words online (such as "cookie"), downloadable as separate GIFs or as ZIP files.
- abcteach has free clip art suitable for making cards for bulletin boards or flashcards. The graphics can be downloaded as PDFs.