I order them from Cabelas. Go to there website and search for Woold Chunks. 8.99 for ten pounds and really cheap shipping. I have always been happy with them.
BBQ Galore has a Memorial Day sale going on and their chunk wood is $1 off (now $6.99). Shipping on a single bag is nearly as much as the wood but you can get up to three bags for the same shipping ($5.95). The next level would be 7 bags for $8.95 shipping.
Alternatively, you could find somebody with fruit tress and see if they'll let you have the trimmings. I know of a forum member living near Peoria who posts here when he's got prunings available.
Edit: FYI, I just tried Cabela's site and some of the chunk varieties are back ordered. Just thought folks would like to know.
I want to say 10 lbs. I have three bags of the stuff at home but I can't remember exactly. If you're putting your average amount of chunks on your fire, one bag will probably last for probably three cooks.