You can actually slice up a whole branch in no time and they don't all have to be the same width or length, as you can see, some of mine have little knots on the side which I love.
My DH liked the ones with no bark but I loved the ones with bark so we did both.
After you slice them drill 2 or 4 small holes and lightly sand.
Soak them overnight in a (clean) margarine tub filled with satin polyurethane or shiny polyurethane (makes 2 very different looks). I like to leave the pins in the holes so they won't get filled with polyurethane.
Next sand both sides lightly again.
Once sanded pin them to a box, a pizza delivery box with sides intact is perfect, then go outside and spray the side facing with satin or shiny poly and let them sit outside overnight. I purchased some steel straight pins (you remember, the kind we learned to sew with? Because you'll be spraying the pins as well as the buttons, the pins are pretty much ready to throw away after you're finished with them. Don't worry a box of pins are inexpensive compared to the price of hand made wooden buttons. I left them in the house to dry since I live in a humid climate winter and summer.
Next day turn them and spray them again. Another light sand, another spray , let dry, spray other side, let dry and they're done. So you've sanded them 3 times and sprayed them twice.
After the first test one was done I sewed one to a swatch and machine washed and dried it 5 times that day while I was doing laundry. The test one came out just like new so I knew we had a winner. I've had my wooden buttons on a best loved cardigan ever since we made the first button back in the 80's. The buttons look good as new although the sweater could use a replacement. I always take them off the garment when I toss it so I have never run out of buttons.
When you sew them on, don't necessarily look at the dimension, try it in your buttonhole, it depends on width as well as thickness and the little knots on the sides, whether or not it will go through the hole so your sweater may have 5 different size buttons.
BTW, after you have finished spraying them with the poly, you can get some acrylic paint and a small paint brush and paint flowers or toys or whatever you want on them, spray that after its dry and you have a button that will match any theme and they look very different from the plain ones.
The picture above is just one of my buttons. We tried several different kinds of tree limbs and each one looked different. I think it was ash that gave us a larger lighter color. We also used maple and cedar. Cedar were my favorites.
Try a branch, I'll bet you love them.