Greetings Mark-
We always used to make our own "seamless" tambour sheets. I will tell you how to do this in case you do not find a source of pre-made seamless tambour.
First start with a piece of 1/4" ply or mdf core in the specie you desire. This will need to be 3-4" larger than the finished piece you will want to end up with.
Part of what gives the illusion of seamless is the continuity of the grain within the piece you are using. This is achieved by arranging the strips exactly as they come from the saw as you are cutting the individual tambour strips. To help keep the strips in the proper order,
we would draw a triangle lightly on the face of the material, if the strips are arranged correctly, the triangle will re-appear as a whole triangle (Minus the saw kerf) When the strips are arranged properly, take masking tape and tape the strips together on the face side.
You are now ready to glue your strips to the tambour cloth. For tambour cloth, we always used denim or light canvas type material. This is available at any sewing store. We would cut the cloth a few inches oversize, tack it to some cardboard, and spray it with contact cement. Take the sheet of strips which have been taped together from the face, and spray contact cement to the back side. Stick the denim material to the strips. Once you have done this, you may remove the tape from the front of the tambour. You will now be able to flex the tambour sheet in the manner that is needed for rolltop desks etc. If you leave a few extra inches of canvas, you can also glue a leader strip for the placement of handles or pulls. I will assume that you already know how to make the track with a router and a template.
I hope this helps!
Best Regards, Chip Geffre