The SRF04 works just like the vex sonar except for the timing parameters. Vex didn't publish the specs but, you may get them from the IFI sensor test program and modify for the SRF04. This site has the SRF04 for a good price.
http://www.hobbyengineering.com/SectionS.html
If it's for a VEX competition you have to use VEX parts. The radio shack web allows searches for stores that are in stock. Our mall store has 1 sonar and 2 encoder kits left as of 1/3/2006.