Use common courtesy when asking for boxes, if you say you’re going to show up then show up. Get people’s cell or email so you can let them know if something comes up.
If you need bigger boxes you can look on Craigslist or your area Facebook groups for someone offloading their moving boxes cheap.
Some U-Haul locations have a spot where people can leave their boxes, and you can pick them up free.
McDonalds fry boxes are great boxes, they are strong, they stack beautifully, they are almost always clean, they are free. Go inside and ask for boxes, they will usually tell you there is a cardboard only dumpster in the back to look for more.
Liquor stores are excellent sources for boxes. They usually keep a pile of them in a corner until the end of the day. Ask and someone will let you know if you can take them, they keep some empties for boxing purchases that are not for taking. The boxes do tend to be smaller and sometimes are cut open funny, requiring more taping.
Most big box stores have compactors now and don’t have boxes unless you happen to catch some. Mom and pops and restaurants often have boxes, but it is better to call around and find some, than drive around hopelessly wasting gas.