For software interaction design, it would be worth checking out CHI groups.
There is a listserve called CHI-JOBS that has daily job postings for software interaction designers. They could easily post a message there and reach thousands of designers.
They could also post something to OK/Cancel’s job site.
Also Dice has lots of software positions. I’m not sure if it’s free to post a job up there though.
To find a firm, I think the best bet would be to get references for local places that specialize in this work. If it’s around San Fran, PM me and I can refer a handful of small outfits.