"Best way" is very dependent on several different things. Your location is a big one, because that could put you very close or very far from a good source of the lumber needed.
What kinds of tools do you have available.
How much are you willing to spend vs. how much time are you willing to work at it?
For me, my dad and I own a mill and he owns 280 acres of woodland. So for us it's a matter of a couple days to cut down a few trees and mill them to our needs.
Not everyone knows someone with this kind of equipment, BUT there are lots of small business people who do have their own mill and are willing to cut lumber to your specifications. Of course then you need to dry it, which you can either wait or pay to kiln dry it.
Of course you can re-dimension it like you are talking, resawing 2x4's and such (Personally that would be my option of last resort for the quantity you are looking for)
A lot of stores sell pine boards pretty cheap and the 1" boards are really 3/4" or 12/16" and running them through a planer or even better a 4-sided planer for T&G might be the better option than cutting down 2x4's.
Personally I would first look for small lumber millers in your area and see what they might be willing to do for you, if nothing is nearby or reasonable, then check the cost of buying pine boards, I assume you already have a way to make them T&G since you would need to do that with resawn 2x4's. You might even be able to buy 8"x 1" (7.25" X 3/4") pine boards, you could rip them in half for 2 boards per. ( appears that 1x8's are a bit more expensive than 2x4's so cost would be higher, though less work)
Otherwise you can look for farm auctions and such where people are selling large piles of lumber for cheap.
