I know that this is an old topic, but I thought my sourcing might be useful be useful.
I get my lumber from the scraps of 2 local cabinet makers. They have high quality materials. Now I don't get long beautiful boards and have to make a couple more cuts, but the cost is right. A gallon of my honey each for a couple of pick loads.
Lately, they haven't had much, so I have been asking local farmers if they have any barn board they want to get rid of. Again this is usually pretty high quality, but I have to run it through a planer.
Also, when I make my boxes, I never paint. I rub them out with paste of beeswax, linseed oil, and a bit of turpentine. This seals the wood really well, never comes off, waterproofs the box, and cures into a pleasant golden color. I make gobs of the stuff, package it in little plastic tubs, like car wax comes in, and sell it to local woodworkers.
Last, I think the bees like the smell of pine and beeswax, because I have caught several swarms in these boxes with no baiting other than the finish.
Anyway, forgive me for reviving a dead topic.