A couple years ago I started researching minor league no-hitters using newspapers available on line from Google and the LOC Chronicling America, as well as Sporting Life and The Sporting News through Paper of Record.
I started with a spreadsheet of the no-hitters listed in the 3rd edition of the Encyclopedia of Minor League Baseball. I got the spreadsheet from another SABR member, who had added some games that weren't listed in thr EoMiLB.
To date, there have been more than 350 no-hitters not listed in the EoMiLB added to the spreadsheet. I've been sending new finds to Lloyd Johnson, one of the EoMiLB editors, so they can be added to the next edition, which is in progress.
Anyone who wants to peruse the spreadsheet is welcome to do so. Here's the link: