Sometimes my Costco will have a Sun City pass. It's tickets to DL and a couple other SoCal places like Sea World. Not all the time though I it's never online. It's something you'd have to go into the store to see if they have it.
No, they don't typically sell tickets only, unless there's a So Cal ticket pass (last time, I think it was $199 for a 4-day hopper with $50 gift card but if you wanted to upgrade, the value of the park hopper was only $151).
What Cosco offers varies. It also depends on if you are in the West or not if they have things at the actual stores for DLR or WDW. Tickets and package offers can come and go.