You're checking all the right places. Pricing on varieties is fairly esoteric, so few mainstream resources unless they are in the CPG. I notice you did not list CPG in your resource list so it would also be useful to have, but note that a new edition is coming out soon so you might want to wait...
Try to not pay too much premium for their being slabbed. Most top 100 RPMs, even the FS varieties, are still raw and can be had for prices closer to the price guide values or less.