Around where I live, the restaurants almost all throw their empty cubies away. Some separate the plastic from the cardboard; some don't. I suggest finding a friendly restaurateur and asking him or her to save some for you. In any dealings with restaurants (or other people for that matter) try to make things easier, not harder, for them. For example, most restaurants have very little or no extra space around the kitchen, and most cooks have little spare time, depending on time of day. If you can arrange it, pick up the cubies on the same day they're emptied so they don't sit around in somebody's way. One place I was getting oil changes their oil at a very specific time twice a week. I just showed up when the cooks arrived to start their day, with my metal containers (Hot oil WILL melt cubies) and they were happy to run the old oil right into them. That saved them the rest of the procedure they otherwise went through to get the oil into the tallow company's barrel.
Or just nose around the dumpsters behind the restaurants. It's a good idea to talk to somebody from the restaurant first so you're not reported as "loitering" or something like that. If you're going to travel from fryer to fryer as you plan, you'll need to get used to approaching strangers - some of whom speak little or no English - to ask them things they may never have heard of.
By now, you must have read at least some of the stickies on the oil sites about stuff like "How to get oil".
Steve