Either a stock vector or really high res raster...I like istockphoto.com or shutterstock. I did a couple wraps to make modern vehicles look like the old woodies. My technique was to find a really high resolution wood grain from Istockphoto, make it tileable (can be done with any image, lots of tutorials out there), then once I had that I would use layers for hue/saturation, color balance, and levels adjustments. I would then create a swatch from that in Illustrator. The end result was a non destructive fully controllable wood grain that I could use as a fill on any vector shape. If you set up the tileable image with care then it won't have any redundant, repeating pattern. And if you aren't happy with it looking repeated from the tiling then you can go back to photoshop and instead of making a tileable image, manually tile it, and any areas that are redundant just use the clone stamp tool to sample another part of the wood grain and paint out the repetitive areas. If you aren't proficient and photoshop and illustrator there will be a few learning curves, but once you know your tools/software this process happens in a matter of minutes. Hope it helps
