Reliable substrates?

I’m wondering if anyone knows of any reliable substartes for the land section of my tank? I think it would make any turtle feel more at ease standing on a naturual substance rather than a slab of plexiglass. It can’t be gravel or anything that holds a lot of heat because the ceramic heater is right above the land area. I obviously don’t want to roast the turtles. This is pretty much the last optional step until I have a fully functional tank, so while I’m on the subject, does anyone have any Terrapins for sale? I’m interested in Northerns especially, but not hatchlings. I think they’re a bit too fragile for my tastes.