[Alex W]

Second Wave (secondwave.cc) is having their final clearance sale in a few days. They were the best place for used stuff in Seattle, but recently closed due to losing their lease.

The next closest options are in Poulsbo, Bellingham, and Portland.

I'd buy slugs new, they don't last forever and aren't that expensive. If you belong to any boating club or organization you'll get a discount at Fisheries Supply and pay 52 cents each. Without they are only 68 cents each. Even if you find them used for 30 cents each you'll only be saving a couple of dollars.

 

[Alex W]

Defender is selling the Ronstan:
Ronstan Sail Track Slide

Fisheries sells that and the cheaper Bainbridge:
https://www.fisheriessupply.com/bainbridge-international-round-internal-track-slide
https://www.fisheriessupply.com/bainbridge-international-slug-slides

In general I've found Fisheries (with club pricing, which again, almost any sailor in Seattle can get) to beat Defender on prices of just about everything except for electronics. They also seem to have a wider range in sailing items than Defender does.

 

[Alex W]

The slugs that I linked to will work just fine in tracks that those worked in. I've never seen ones exactly like that, but those could also be 40 years old if they are on a C-Lark. If you have 20 of them on a C-Lark mainsail then the cheap all-plastic ones will probably work, but you might want to get just one first to see if it fits into the mast track.