I used an old mirror that had two edges that were broken. Your local hardware store would carry various mirror sizes that vary in cost. The size of my mirror could be purchased for around $50.
Wood stain of your choice
Below is a before picture of the mirror. It was a 5 ft. X 3 ft. mirror that was broken on two of the edges. After purchasing the materials and having my local hardware store cut the wood, I placed the mirror on the floor and the would be wooden frame over it to assure a good fit.
Made two Jig holes on each side of the two 1 in. x 4 in. X 5 ft. wood pieces. Then, added glue to each edge before using screws to connect them to the 1 in. x 4 in. X 3 ft. wood pieces.
Stain the wood to your liking. If you are trying to match a color or want to know how to protect the wood, refer to Staining & Protecting Wood.
Add mirror adhesive to the mirror edges using a caulking gun. Lastly, add the mirror clips.
Here is the final result.
If you decide to give this a try and make your own rustic, farmhouse mirror, reach out to me via Instagram. I would love to see a picture!