I'm missing presets in InDesign. Are there gradient libraries somewhere that I don't know about?
Not for gradients, I'm afraid. You need to create your own gradient swatches.
Do you know if there is a place online or in Photoshop/Illustrator/etc. to import existing gradient libraries?
I don't know about online. There are gradient libraries for Illustrator, but there is no way I know of to get the swatches to move from AI to ID. .ase files don't support gradients or patterns, and pasting a gradient filled object doesn't seem to bring the swatch, either.