I found some seamless tiles, some spherical textures like IBL, and some panoramic (none equirectangular) skys at Flicker.com. Also, my runtimes has some seamless tiles of ground and grass.

I have a lot of seamless textures on my hard drives that I can look through. Some might work for this. I believe Poser World has offered some in the past. With a target product like TerraDome for Poser, they might just offer some more high resolution texture maps of rock, ground, sand, snow, etc..

LMK

Probably edited for spelling, grammer, punctuation, or typos.