I went to the local copy center (Kinkos or similar) and asked for empty copy paper boxes they were going to toss. They are similar to banker boxes but had held reams of paper. They gave them to me for free. I did reinforce the ends with packaging tape though. If you wanted them to look better, you could cover them with fabric or maybe modge podge something pretty on them.
I didn't get any (but should) but I know copy centers also have smaller boxes that use to hold envelopes, stationary, business cards, etc. They would work well for storing smaller items or as project boxes.
I have a friend that works at our local book store. When I needed cardboard put on the ground to kill some weeds in a garden project, she said I could come get some from their recycling bin. Most of them are the same standard size which would also work for storing sewing supplies if you label the outsides. Unfortunately they have flap tops instead of removable tops.