I got some of my flyers on ebay and some by doing searches on the internet.
BTW: I saw that user MFFE has about 800 flyers and schematics up for auction this week. They do have some lots, but most are individuals. I have purchased many things from them in the past; good people to deal with.
I have a good 300 arcade flyers in my collection. I got mine from my local arcade game distributors. I went to mine usually one a week when I was at one my old jobs and it was down the street where I had to pick up my supplies. Don't go to the ones that do repair and supplies the liquor stores and laundromats. Go to the ones that sell or has access to the new games, pool tables, dart boards, jukeboxes, and of course pinballs and video games