Schlappen no good. Saddle hackles are what you want, preferably un-strung. You should be able to find some good usable skinny saddles on patches or in packages of saddles. You don't need the real long (expensive) flatwing hackles because the flies you will tie don't need to be long. Besides, flatwing saddles are hard to come by and expensive now.
You are looking for saddles say 5 to 7 inches. Ideally, saddle patches would be best but the packages of saddles by Hareline or better yet Spirit River are pretty good. They come in UV too. Often, you will come across strung saddles. While not the best, they are readily available.
Photo is of some strung white saddles. Spirit River brand .
Also, a photo of an ideal saddle for a flatwing which I pulled from the strung saddles. All of the saddles in a package may not be suitable but there will usually be some good ones you can use for flatwings. Obviously, you will want other colors besides white.
Hope this helps. You will have to visit some fly shops and look at their saddles and pick out the ones that look best for flatwings. Try Waters West or Sportsmans Warehouse.
First inspection these saddles look better than I expected. As you might imagine Jack the only flies I've cast since we fished are FWs. Afterfinding a candle fish on the beach my most recent patterns are about 5" long and highly effective. I recently added a split shot at times to get a little different action.
Glad you found some saddles. Not all saddles in a package will be suitable. When UV is available DEFINITELY go for it. UV is a good thing!
Five inches is probably as long as you want to go. That's why expensive long flatwing hackles are not necessary. You would be wasting an expensive 10-15inch long hackle just to use the last 5 inches or so.