I have used cups, and yes they fall over, which is a pain.
Inserts that fit into a 1020 web flat come in every arrangement. I ordered "sheet of pots 18," which is 3x6, 18 pots to a sheet. They come all attached as a sheet, which makes watering easy for me, because I dunk a flat at a time in a bin of water. BUT before you take them to market, you have to cut apart the sheet of pots, which is a huge pain and time-waster. I tend to get potting mix on the crease and can't see where to cut. After they are cut, they are only a little more stable than the cups. They still flop everywhere when I try to dunk them in water.
The generic brand of solo cups in the big pack was I think about ten bucks at Sam's. It came out to about four cents a cup. The sheet pots only beat that price by a tiny amount, about half a cent.
Hummert is the supplier I use:
http://www.hummert.com/ProductList.a...tle=Containers