If you go on the Kerbal Maps OP, there is a link to a mega repo, that has a lot of planet textures (but their from 0.21) in 8k, my laptop didn't like that so i scaled it down to 4k, but it should work perfectly, he also provides heightmaps, but their format is a special type of tiff(not sure what, but he explains how to view them in to OP). There is also this, but I haven't looked in to it much, Ill test those ones out later. <-- these are not very high quality.