It may seem dull at first sight, but this little book of finger excercises is really useful, and promotes stretching between the inner fingers without breaking them. I use it myself a great deal. It is by Anna Priscilla Risher (1875-1946) and is unusual for being written solely for the white notes (there is no instruction to transpose), and maybe that's all one needs to fulfil this particular technical aim. Here is more about Risher: http://www.parlorsongs.com/bios/aprisher/aprisher.php
And you can download the studies from here: https://eu.storagemadeeasy.com/files/f562ccd59dc614db20eb750fb56517f1.pdf
. I will put them on IMSLP, but they will only be available on the EU server, I guess.