I bought my laundry bags from bed bath and beyond for pretty cheap but you can make them pretty cheap too.
The bags are sturdy cotton and are roughly 32 inches long and 27 inches wide. They have a drawstring in the top and a little spring loaded barrel tie. They're nothing fancy.
If you were to make these, you would need 2 panels 35 inches long and 28 inches wide. You would sew both panels together around 3 sides approximately 1/2 inch in from the edge leaving ond of the 28 inch sides open. You would then fold the edges of the open side down 2 1/2 inches and sew around the circumfrence of the opening 1/2 inch in from the edge of the fabric (2 inches in from the fold if you are so inclined. After that you turn the whole thing inside out and cut a little hole to thread your drawstring through and get to threading.