I'm puzzled why there are so many "Looking for free plans" types on this list.
The cost of the plans for a project, regardless of what it is, probably represents less than 5% of the total project cost.
If you truly want to reduce your costs, reduce the price of materials by buying quantities and with terms the supplier can use to offer you a lower price.
Invest in machinery to reduce the time to complete a project.
Invest money to reduce your energy consumption costs.
Find more efficient ways to minimize your cost of money, if you borrow it.
Any or all of the above will reduce your costs far beyond the few $ you might save with so-called "free plans".