Nothing sells out, I've been there to the party in late September and early October and they have always had plenty of everything.
Same on tickets. I believe a few of the late October weekend nights can sell out, especially on Halloween itself. But if you are going in September you should not have a problem. Every single September party we've been to (3 or 4), the park announcers were hawking tickets over the loudspeakers they normally use for parade/firework announcements, trying to get the day guests to buy party tickets and stay for the evening. So even at the last minute there were plenty, that early in the season.