Aside from the pics route
, for the original track try:
Nicholson Owner's Associations/Groups
Camper Nicholson - As they're still in business
(The) Boat's Spar Maker
Riggers - for; the track, or pre-loved spars needing new homes
Yards & Marinas
- Same as Riggers
There's also plenty of track still made for various types of old school mainsail
slugs & slides. Some of it's screw/bolt on, but if you don't mind swapping track styles it's an option. And there's also lots of track made for other varites of other types of older style slides. Most of it's stainless, which probably makes it easier to fit to a mast
from the perspective of joining the sections (via welding).
Check with the various Spar, & Yacht Hardware
companies. For some reason Ronstan comes to mind, but there are scores of them.
Also, if you were to put up wanted ads on various bulletin boards; paper & electronic. Then I'd wager that you'd get more offers for track, probably gratis, than you could handle. Just be sure to put the dimensions, specifics, & a few pics in the ads.