Woodworker's Supply (800-645-9292) sells Moser's orange shellac
flakes at $14.95/pound.  They also sell four other grades.

This works for sealing channels, but I also use thinned white glue
and also waterborne lacquer.  Just seal off one end of the channels
and let the stuff soak in for a while, the drain.  I'll blow through
the channel with a tube just to be sure it's open.

Philip Jamison
Pennsylvania