You can easily make it from 30-06 and possibly 270.
Maybe talk to these folks and see if they have it.
https://opticsandammo.com/product/hornady-case-35-whelen-50-mfg-87291/
You can make 35 Remington out of .308 brass but not .35 Whelen, you need 30/06 brass for that. And yes the necks will be thinner but it works and it’s cheap enough you can throw the brass away after you shoot it. Just make sure you make the brass so you do try to put it in an 30/06 gun. It wont fit as a loaded round and you won’t want to use the brass for reloading as a 30/06 round.
Bryan