Butterfly flexibility recommendation

At 61 years old, I’m not much of a competitive pool swimmer, and consequently don’t swim much of the ‘other’ strokes. When the open water warms up enough, just about all my swimming over the summer months is in the open water (swimming only crawl stroke). Once fall comes back around, I’m back in the pool…and I like to add a bit of the other strokes to my yardage. This fall, though, as I’ve tried to do some butterfly (which I’m not great at to begin with), I’ve found my shoulders to be less flexible, and having difficulty/pain just getting my arms high enough out of the water. Nothing wrong with the back/breast/crawl strokes. I think I just need to do some stretching and flexibility exercises. Can anyone recommend something?
Dan