Official sources for these images can be found at the Federal Highway Administration's website in their section on Standard Highway Signs. Detailed specifications are also shown within the MUTCD manual. The background color yellow is specified as pantone 116. Some of the newer signs use diamond grade fluorescent yellow-green background. Yellow-green signs may be seen on school signs, playground warnings, pedestrain crossing, and others.
Places like fonts.com will sell commercial packages of sign symbols like the ones shown here, search for highway signs on their site. I don't use these packages, so I can't comment on the quality or usefulness.
"The Manual on Uniform Traffic Control Devices, or MUTCD defines the standards used by road managers nationwide to install and maintain traffic control devices on all streets and highways."
Warning signs shall be designed in accordance with the legends, sizes, colors, and shapes found in the "Standard Highway Signs" book. The use of symbols results in faster recognition times compared to worded signs which takes longer to comprehend. Another benefit, symbol signs can be recognized at farther distances which gives more time for the road user to react to the sign.
Wikipedia has a nice section on warning signs and lists additional graphics.
Disclaimer - This website is not an official source for road sign images. I'm not affiliated with any federal or state agencies and do not warrant that these images are accurate or free of errors.