The bike shop I used to work at, over 26 years ago, sells boxes for $5/ea, which I have purchased in the past. Driving by a newly opened shop only 2 miles from the previous, I see a stack of boxes outside one day and immediately do a u-turn. While I am outside checking out the boxes one of my, previous to Covid, employees pops out and says she is working at the shop, and the boxes are free! I stuffed the car with the best three and was stoked to save $15 in the process. If the market picks up and I can sell a few more bikes, I will be going to the free bike box store from now on!
Or you could buy these crazy expensive boxes from BikeFlights.
Or these from Uline.
Full-overlap flaps provide extra stacking strength and edge protection. Recommended for artwork, mirrors and other flat or narrow products. ULINE offers over 41,000 boxes, plastic poly bags, mailing tubes, warehouse supplies and bubble wrap for your storage, packaging, or shipping supplies.
www.uline.com