There are a lot of different sets of textures which you can use, but Totally Rad Dirty Pictures, and Flypaper Textures seem to be two of my favourites. There are also freebies, and you can also create your own, if you have the time. There is another site called "Texture A Photo" which was featured on a Martin Bailey Podcast, but the bank only goes so far.
You can download it or have it sent on a flash drive.
Totally Rad will add your existing textures to its' library and autofit and rotate the textures to your base image. You can also preview your images with your selected texture, this is a huge time saver! The textures are also in RGB colour space, at 8 bit. If you work in TIFF (16 bit), I just convert the texture to the working space I am in when in Photoshop CS6.
The photo above is a Wisteria seed pod, shot high key on white matte board for the base image. Using Live View gives you a great preview on your camera for exposure, white balance, etc., (Canon cameras) before you press the shutter button. I added lots of different textures, and I will often use poster edges in Photoshop filters as well, to give an even grittier look. It is a lot of trial and error, and I have been working with this technique for about two years, and I can usually get an effect that I like and have in my head. A lot of times too though, it is a work in progress and takes shape as you are working on it. The image above had about 16 layers or so, all with different opacities and blending modes. I think this image took about 3 or 4 hours to complete. It is one of my favourites.