DeviantART has a lot of nice textures, just search "Textures" and be sure to read the rules.
Some people think it's perfectly ok to use the textures as you please, other want credit and some don't want you to alter or edit the textures what so ever. So be sure to read and respect what people would want you to do with their art.
You can also make your own textures. If you got some water colour paper, scan it and edit that in... Per say, Photoshop or something similar editing program.
Wrinkle a paper, add coffee stains, your imagination is the limit! <3
Adjust the lights, (Photoshop for an example) by using ctrl+l to adjust the levels. Make it lighter, darker, bring forward the shadows.
You can set the layer then to multiply, soft light or overlay to get a nice looking texture feeling to the artwork.
If you want to adjust the colour on said texture, use ctrl+u to adjust the hues.
I've scanned fabric to use on my artwork, as well as lace, scrap booking paper and just about anything else I could find with a texture I enjoyed.