H&S,
I'm going to make two suggestions.
I found good sticks and sheets at building sites. They usually have a fenced in bin where they throw their waste and scraps. I got quite a bit good fresh wood and PVC pipe scraps at these sites. Just look for new homes and commercial buildings, talk to the manager and worker bees and get their permission to pick thru their waste and salvage what you can use. This takes time and many outings... but the price is right.
I've also found it useful to check demolition sites, especially really old buildings. Check with the owner or demolition crew and ask if you can have some of this old lumber. You may also want to consider purchasing this wood if they will not let you have it for free. This is really seasoned wood and the dimensions of this wood will surprise and please you. You may end up with odd lengths, but if they are long enough you can re-saw them, remove nails from them, scrape cement off of them etc.
I used the 1st method to get quite a bit of dimensional sticks for two of my benches. Of course I had to supplement what I had with sheets stocks from the builders supply as well as hardware supplies. But, you can save a lot of money by being a good scrounge and bargainer.