Over the weekend I needed to do a preWWI Magazine cover. I really just needed an image, since it's a made up magazine, but I searched covers in attempts of narrowing it down. Magazines are something I find I quite commonly need to do for "business" hand props. So normally you can just pull a period front cover and a random period ad and size it to fit over a current magazine or fill with Ledger size paper folded in half.
I found a TON of good resources, some which I've used in the past. I did quite a bit of searching through most of these, so I hopefully will be able to tell you what's there.
I did a lot of searching through this link in hopes of finding something. Other than being a pain in the ass to just browse through, it's pretty great. There is a large range of years and various magazine covers. The image sizes are fairly big. I couldn't find a "main page" that had the list of links for each section like the bottom of the page has, so it's linked to the 1917 page.
This flickr page didn't have quite what I wanted. I think it's mostly stuff from the teens and twenties. I believe it's all cover art by a specific artist. There are 340 photos (as of today), so it's worth taking a look at when you're searching. The nice thing is that you can hover over the thumbnails and see captions that tell you the magazine title and publishing year.
The next link is to a blog entry. There are only about 40 covers here according to the blog, but they have a good range in types of magazines and years (1913-1988). If you are looking for some sweet covers, I recommend looking here.
I found the cover I used at MagazineArt.org. It has a nice large collection. My only issue with it was that I couldn't look by year, only by title. The nice part with the search is that you can narrow it down by category then title. It's also nice because you can scroll through each title if there are multiple images available and you want to view all the large images. It also has ads available, which is handy for making back covers.
I used Whodatedwho.com last summer when I needed gossip mags for South Pacific. If I remember correctly, the images are a little small and will pixelate a tad when you blow it up to magazine size, but it's nothing that is noticeable off stage. There are a great amount of movie and gossip magazines available. You can search by year and page through the years. It will also give you the option to narrow down by magazine title within a year. I haven't looked at this site for much else, but it seems that you may also be able to get movie posters on here dating back to 1913. Oh, one last thing, the earliest link date is 1930 for the magazines, but there are some available further back than that. Just click on 1930 and page back.
Finally, if you want vintage Good Housekeeping covers, there are some available on the website. I haven't looked at it much more than seeing that it was there. I just saw the link when I was searching and thought it would be a good one to bookmark.
I'll add these links to the sidebar in a bit. Right now I need to get back to work. I'll work on getting some more links up tomorrow before I get off for another long weekend.