Where can I find/make a handbill like the one that brings the Joads to California in The Grapes of Wrath?
Unfortunately, precious few examples of handbills survive. A selection can be found in the historical archives of Oklahoma State University, Stanford, and Princeton.
However, Steinbeck's documentation in The Grapes of Wrath comes directly from handbills he knew to exist, the wording voiced by a disillusioned migrant worker who tells the still-hopeful Joads:
Now, how many of you all got them handbills?... There you are, same yellow handbill. 800 Pickers Wanted. All right, the man wants 800 men, so he prints 5,000 handbills and maybe 20,000 people see 'em. And maybe two or three thousand people start West on account of that handbill. Two or three thousand people that are crazy with worry headin' out for 800 jobs. Now does that make sense?"
Although Steinbeck never traveled to Oklahoma, he tirelessly researched the plight of the "Okies" in his homestate, California. The "Tom" of the dedication of Grapes ("To Tom, Who Lived it") is Tom Collins, the real-life director of the "Weedpatch Camp" where the fictional Joads find refuge. Check out the pamphlet Steinbeck created with renowned photographer Dorothea Lange, Their Blood is Strong for examples of how you might word your reproduction and get a feel for the era.
The third link will take you to "myfonts" and help you create a proper Depression-era typeface. See second link for a selection of Lange's work with the Oklahoma migrants in California.
The search was not easy, but I found you a gif image of a flyer advertising the need for 5000 cotton pickers in Arizona. It's not California, but it is a job posting that would have led many "Oakies" to leave their homes for work elsewhere.
That was the only photograph I was able to find, but I hope it helps!