Oh, yea, the main source of those grimoires for SSRs will be union events and post-update advent events (that is, starting with Illuyanka in a couple of months or so). Also can be exchanged for with eidolon orbs and tower medals, but you're saving them up for other things.
Or if you're really in a hurry, you can trade for some in the upcoming reprints before Illuyanka, but the prices will be higher. It's -/300/225/125 to pick up 5 of those grimoires from something like Qiong Qi or Sphinx reprint compared to -/250/200/100 for 10 grimoires starting with Illuyanka.