I work in a kitchen, and I take these home all the time. I've had no issues using them with my feeders, even the ones that do have some minor grease or egg spots on them.
I also take the empty saran wrap rolls from work. These are nice to use with bugs or feeder rodents, as it has really thick cardboard that can be chewed on (by rodents) and makes good hides as well.
I also go to the local fabric store and take some of the empty cardboard tube rolls that used to have the drapery fabric on it. I cut these down to size, they make good hides for feeders and rodents, they also act like the cricket tubes from the cricket keepers.

Lots of kitchens will have an assortment of pails from left over foods (fruit, butter, oil, sauce, etc...) some of these can be handy to use too.
Larger 5 gallon pails, if you collect a bunch of these, (6?) you can make a sorting device by drilling the bottoms of the tubs with different sized drill bits. It makes cleaning go faster and getting the sizes you need quicker too.
Left over salad dressing containers, spice containers can be used to store bug food, and water gel in.
You can cut pails down to make food and water gel bowls.
A bag of Chick egg laying mash from our local feed store fill about two 5 gallon pails up. So they are good for storage too.

Keep in mind that kitchens have to pay every time they get their dumpster emptied. It's not cheap either, so you will be doing them a favour by recycling their "trash" for them.