etsy. Antique malls for old magazines. When I was searching for Lionel train ads several years ago, I would go to the public library to peruse old Life magazines for ads. That then gave me issues to seek in the wild. Look around November-December for the Christmas ads.
some used book stores have old magazines in them. National Geographic, Life, etc. Unfortunately, the older they are, the more pricey they get. In some cases, may not be as cheap as going through ebay for just a single ad.
We find old Life magazines at estate sales where they are reasonably priced. I spend the first pass through looking at the ads. All of the vintage ads are interesting.