Go to Lowes or Home Depot, go to the plumbing section, and it is sold in a roll about 4 inches in diameter and 1 inch wide. It is grey in color. Ask an associate for help if you cannot locate it. (Good luck finding one). :/
You will need two 1/4" bolts about 1 inch long, four 1/4 washers, and two nylock nuts to attach it. On each bolt assembly place one washer under the head of the bolt and one under the nut, metal strap sandwiched in between. The washers are necessary because the strap is soft metal and the washers prevent tear through over time due to vibration.
You wrap it once around the muffler and select the holes that will allow the bolt assembly to squeeze it tight.
Next, take the loose end and wrap it around the bicycle frame, routing the metal strap in a straight line, no bends, and select the bolt holes that will allow the bolt assembly to squeeze it tight.
Edit:
Here in this thread of a different subject,
http://motorbicycling.com/f4/i-think-im-missing-piece-my-30698.html the OP has used a much stronger metal strap still sold at home improvement stores, but used as reinforcement for roof trusses, wall studs, ect. It will hold up much better than the plumber's tape but you will need to drill your own holes and it will not be as easy to bend around the muffler can and bicycle frame.
As a last thought, there are pre-made hanger straps sold in home improvement stores that are pre-bent for supporting water and sewage pipes.
They look like an outline of a lollipop.
They usually are held together with a single sheet metal screw though and that screw needs to be replaced with a nut and bolt. Also you would need 2, one for the muffler and one for the frame, bolted together in the middle. You would have to search for the best fitting ones as they come in different sizes.