It sounds like what you are looking for are "bite tippets" to attach to your regular leader for fish with teeth or gill plates that would cut through a regular leader. Is this correct?

If so, you can buy mono or fluorocarbon tippet material on spools in 60 or 80 pound test from many retailers, such as J Stockard (sponsor of this forum). This heavy tippet can be difficult to straighten, which is probably why you have seen it packaged in short segments in tubes. You can straighten it easily by putting short segments in a metal tube and leaving them in a warm (120 degree) oven for a few minutes.

Ted