The I.D. of this bottle is 28 mm which would equate to a #6 or 26/32 joint for the stopper. See the links below to review two options.
BR144391, pack of 25 #6, 26/32, solid red rubber stoppers
https://www.sigmaaldrich.com/product/aldrich/br144391.
Z165344, set of 66 assorted black rubber stoppers, including 1 and 2 hole options. sizes range from #1 to #8
https://www.sigmaaldrich.com/product/aldrich/z165344