Funny you should ask this, since I just ordered a hot knife blade for my Weller gun yesterday. Found one at Quality Dist, through Amazon for $5.95 - Much cheaper than buying a dedicated hot-knife tool. LINK HERE.
Aside from searing rope ends, my main purpose is to cut the raised polysulphide caulking joints flush with my teak decking. I've never tried this, but several people have told me it works well.