In the States, you can go to a 'bike store' and ask the mechanic for 'star nuts'. You will want to take some careful measurements to find nuts that are slightly larger than the inside diameter of the tubes (I've found '1" star nuts' that were anywhere from 7/8ths to 1- 1/8ths). You can grind them into a square shape, or-- much simpler-- shape them into rough squares with a hammer. If your bike shop is like mine, you can expect to pay anywhere from $0 to $5/nut. [EDIT: I was remembering paying $2/nut, but reading the thread gummi linked to reminded me that I've paid up to $5.]
The reason you want to get the size pretty close is that the way you install them is to thread them onto a bolt and pound them into the end of the tubes. If they are too small, they will pull out. If they are too big, it will be hard to get them in straight and you'll risk bending the bolts. In extreme situations, too-large star nuts will even deform the square tubes on your SS.
Oh, and when pounding them, make sure you set the other end of the tube on a block of wood or other similarly soft surface, so you don't damage or deform the other end.