There's a Heathkit manual site at http://www.circuitarchive.com.uk/heath.htm although they don't have your particular manual. You can post to their Heath forum and cast around for one at http://pluto.beseen.com/boardroom/q/51675
Also check out the "Q &A" column of the February 2001 issue of "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines) for a list of Internet sites that deal in manuals. Some of this same list can be found on a posting here at http://www.nostalgiaair.org/nostalgiaair/messageboard/newforum/840.htm
There's still fertile ground out there my friend!
Dean