Yuru Yuri (ゆるゆり) & Oomuro-ke (大室家)

I'm sorry to say but it is indeed a pinafore dress. A pinafore dress is just a dress you wear over shirts or blouses. They are very prominent in slice of life anime but in real life, not so much (at least where I live). The Japanese took this from western culture just like the sailor seifuku and held on to them, but in the west itself these types of things are considered to be a bit old school, out of style. This might make them hard to find in shops.

If you can't find a good one, maybe you can make one for yourself as a sewing project. I have seen people on youtube making them. I believe they are popular in the west with people who are into vintage styles.

Also according to Wikipedia it is called a "jumper" in American English so if you are in North America that term might yield better results for you.

As a man they are one of my favorite garments to see women in but the world around us has quite different tastes and sensibilities. It seems being either "hot" or "hip" is more of a focus in the west and wearing a shirt under a dress doesn't really fit that. I'm sure that in Asian countries you would have a lot more choice in dresses and skirts and other more classic choices for woman, just like a lot more men in Japan still wear suits to work whereas in the west this is very rare.