You can find textures everywhere, at home, going for a walk and all round you.
What do you look for?
Most people take tree bark as their first textures. Tree bark is often to heavy for most images as texture. You want to look for more delicate textures as your basic set.
The basic set should be a mix of textures you use regularly as backgrounds and overlays.
The most important thing is you take the photo when you see. Then you decide what to keep later. Look for textures of any colour. Do close up images and more distanced images. Take several at each texture found.
The textures should be kept either by keyword, or folder by type or colour.