There are a ton of resources online for textures. The two websites I have used in the past are not working right now, so I thought I would show you how to make your own textures. You can pretty much take a picture of anything and use it as a texture: concrete, wood, leaves, grass, fabric, flowers, etc.
In this example I used a picture I took of some hydrangeas. The original image was pink, and I wanted to keep the colors of the image I was texturizing, so I changed the hydrangeas to grayscale. This allowed the colors in the doggy picture to show through instead of having a pink tint to them.
These are two pictures of textures I took in Portland. The sand on the beach was blowing so hard and making these cool criss-cross patterns. It makes a nice subtle texture that I used in last weeks picture of my father-in-law.