Standard five gallon buckets are available on construction sites: paint and drywall mud; restaurants: pickles, coleslaw, and condiments; maintenance and custodial contractors or institutional organizations: detergents, cleaners, waxes, polish. If you have the gift of gab you can talk your way into some for free.

A lot will come for free if your willing to dig them out of a dumpster or remove them rapidly if they agree to stack them up for you. Too often people ask for them to be put aside and then don't follow up so they get tossed just to free up the room. Point being that if your ask and they agree to do you a favor you need to follow through. Basic civility and consideration but it seems to too often get forgotten.

If you need them now, want to avoid banter and negotiation, need them in pristine condition, and are willing to spend some money most hardware outlets sell them. $4 to $6 each seems to be the going price but I have seen them on sale for as little as $2 each.

If you have a few of those buckets you can rotate them as needed. Possibly using them for storage. If space or weight is a concern liner bags might be used. The first time I used peat moss and tried it I didn't use bags and found that there was very little contamination of the bucket. Dumping was quick, easy, and less revolting that I had expected. Bags might be wise with children but I'm beginning to think that they might be more trouble than the are worth if everyone is disciplined and you can dispose of the contents in good time.