If you watch your local Craigslist.com ads for "teardown" sales or "demolition" sales, or look through the Materials for sale section you may find the mill finish style of screen door for sale, and often these still have the grill guards included. It is possible to find new mill finish storm doors -- one company that makes them is Croft LLC, and these may be available through chains such as Ace and Do It Best hardware.
Whether you are buying new or used, you do have to pay attention to the width of your door opening, and which way you want the screen door to swing, because the old doors may not be easy to switch from one direction of swing to the other -- it depends on the kind of door handle that was used and how creative you could be at covering up old screw holes (put a screw in the hole and most people won't notice). For a bit more on finding a new mill finish screen door and parts, see this post at the NoPatternRequired blog, and tell them Uncle Atom sent ya.
I transplanted one of the scroll-work guards (or grilles, or grills, or grates, pick your favorite term) screen door in our front entry. It really gave the door a finished and vintage look. The only thing missing from my entry is the monogram letter R. I have an R but I would have to cut up the grille to make the monogram fit, and I just can't bring myself to do that.
The individual monogram letters typically sell for around $5-15 each, but sometimes less. If you are looking for a particular letter, you can post it below in the Comments section with an email address, and be sure to also check the Comments section of these older blog posts, here, and here. Transactions or trades are up to you to work out on your own.
Good luck with your hunt - these doors and the guards and monogram letters are worth saving and they'll give a nice added touch to your mid century door and entry.