For sure, find a wood distributor in your area. These cater to professionals and might chop up a long board for you with a circular saw so it fits in your van. My friends and I have bought oak and redwood from this kind of place. I got huge chunks of oak; 1" thick is no problem. They'll carry only common woods used for large-scale construction, plus some synthetic materials for decking etc. This will be the cheapest option.
There may also be a boutique wood shop in your area (I've gone to a place called "Wood World". This would be more expensive for such common woods, but cater to the woodworking hobbyist and can (for additional fee) do things like plane it and surface it for you. They'll have a wide variety of "interesting" wood, turning blanks, and plywoods.
I've also bought redwood from places that specialize in just that; they have lumber meant for fences and decks. I've bought 2x4s etc and cut and planed my own stock down from that. My friend bought ugly redwood from the general wood construction supplier; this is to make painted trim, and is cheaper than the pretty stuff sold for fences and decks. I forget what the grades are actually called.
I've bought wood online and had it delivered, dropped off in my yard. Shipping can be pricey, and it's only worth it for a large order or wood that's already expensive.
Go to the local woodworker hobby shops and ask around. Not Home Depot! Places like Rockler and Woodcraft. The sales clerk might know about such suppliers in the area, and even have business cards and a poster display in the store. Lacking such knowledge from a clerk, you might learn about clubs that meet locally, and you can ask there. In fact, for a small piece, getting a cutoff or scrap from a club member might be the cheapest by far.
