If you are looking for ways to make some inexpensive furniture that is study, functional, and can fit with a rustic aesthetic, you may want to think about using pallets.
Pallets are basically large trays that are used for shipping and storage. They are generally made out of wood and can be reused a number of times. They are usually around 48"x40". Sometimes they are just one level of deck boards with stringers that run along the bottoms, but the most common shape is one that has two levels of deck boards with stringers that run in between them. That allows for a forklift or other lift to pick them up easier and move them to where they need to go. They are used by just about any business that does a lot of shipping because they can get a pallet in, unload it, and then reload it as necessary, or stack it up to wait for the shipper to come back. If you are interested in getting pallets, there are ways that you can get them.
Some lumber stores and home improvement stores will actually sell brand new pallets that you can use. They generally aren't all that expensive, so you should have no problem finding some that you like. There are some benefits to buying them new. One is that you have a pallet that hasn't been used before, has no damage or wear and tear, and is still in good shape. If you are going to use it the basis for furniture, this may be the way that you want to go. One drawback is that you are going to end up paying more per pallet than you might otherwise have to pay. There is another way that you can get pallets.
Ask for Them
If you are fine with using older pallets, then there is another way to get them. What you can do is talk to the shipping manager at larger stores, such as big-box stores and department stores. They will get in dozens of pallets regularly, maybe even on a daily basis. As they unload those pallets, they are going to be left with stacks of empty pallets. If a pallet is starting to break down, they may just throw it away rather than bothering to keep it. You can ask them if you can have some of the older recycled pallets. You may have to pay a small fee, but you may also get them for free.
Pallets are useful for a lot of different things, including building furniture.