Free graphs
Every one of these is a graph, not a model — open it in the editor and the parameters that built it are still there to change. Re-colour it, resize it, re-roll the seed, then export to Roblox, Godot, glTF or React.
Free to use, including in things you sell. They are also the fastest way to learn the tool: open one, find the node that looks interesting, and change a number to see what it does.