Surprisingly stacking headers are hard to come by from national stocking distributors. I just checked digikey and mouser and couldn't find anything. I've looked before and I get the impression they are not big stock items.
For Arduino I've always purchased my stacking headers from specialty suppliers. The thing is they are cheaper there than what I would expect to pay from digikey etc. So probably they are getting their stock directly from China via Aliexpress
This is the best I could find so far: 2x15 stacking header The store: Hello Electronics has different lot sizes as well if you want more.
Oh I just realized something. The original Raspberry Pi header is 2x13. Should be lots of those collecting dust. Looking at the expansion dock header pinout it seems that you could center a 2x13 header and still get access to everything you need. Not sure how far that will take you though.