The best deal I've found online is at PetFoodDirect.com. This is where Harlan sells its lab block directly.
Harlan changed the name of it's lab block from "Harlan 2018" sold in 33lb bags to "Native Earth 4018" in 40lb bags when they started selling it online there.
Anyway you can get 200lbs of Harlan/Native Earth there for $80 + roughly $60 for shipping (+ or - depending on where you live) which works out to roughly $.70 a pound, which isn't all that bad for having it shipped to you door. Although I would only order from there if I couldn't find a local feed store that could order lab block for me.
Personally I get 50lb bags of Mazuri 6F for $29 a bag at a local feed store. If you haven't already I recommend going to Mazuri's homepage and entering your zip code in the dealer locator and seeing if there is any place near you that either carries it or can order the 6f for you.