You are correct about the sci application. Actually, we are fortunate that the new stoppers are different from the old, black rubber ones. They were terrible!
But, I digress....if you tell your vendor what size and type of bottle you must stopper, they should be able to meet your needs. If you have an unknown size, there exists a sizing gague, shaped like a carrot, made of thin aluminum, with all the pertinent sizes painted on it. It may be available from some sci supply house. Try LabX.
You insert it into the neck of the bottle, and read off the closest size.
Something to consider: if you see the need for a 2-hole stopper, i.e., for siphoning with gas return, a closed system, ask the vendor if they could drill 2 holes in the stopper, placed where you specify.
I have had success using those orange carboy caps, with the 2 stalks. I use the 2 center ones for the siphon, and the side ones for gas return. That way I do not have air entering the emptying carboy. Good luck.