I think a good first step would be to define your parameters of "low income". That will help you to figure out where to look. If you use a governmental definition that covers the entire US, you will have a very different group of people rather than if you use a California definition. For example, the average family income in the US is around $45,000-$50,000 a year. However, in many of the counties in the Bay Area, the average income ranges around $85,000-$90,000. A family in the Bay Area earning $45,000 may be "average income" for the US, but would be earning only 50% of the area's income and be struggling to get by, especially when you factor in rent/ mortgage costs, food costs, and transportation.
If you go with families making say $24,000 to $50,000, you may be able to locate people through local parents groups. Another avenue to try would be the WIC office. Many of them are run by RD's, so your RD's might know someone. You could try the local school district office or local homeschool groups. (You would be surprised how many homeschoolers are low income, since we usually only have one income.)
Good luck with your presentation and let us know how it goes! It sounds really interesting and very much needed!!!
BTW - Only some of us are comfortable with calling ourselves "low income". Maybe you could "sell it" using a different term, like "nutrition for families struggling to make ends meet in our high-cost area".