Yeah, the late admission discount is great! I would definitely check to see if they still do it. I have not done it in awhile. I have found it's great to do IF the following circumstances exist:
1.) It's not raining out (I have found it to be crowded in poor weather because it is all indoors and people flock to it).
2.) There is at least one park open late that night. If all the parks are closed around 8 or 9PM or earlier (for example, in off season) people seem to head to DD simply because there's nothing else open.
Otherwise, besides getting them from Disney, I have only purchased DQ tickets in person as part of a ticket package from the Sheraton Vistana in Orlando. I was at my cousins time share and they offered a nice little discount on the overall ticket package.
1985-Dis. Village Hilton 1987-Poly 1988-DL
1989-GF 1994-GF 1997-ASMu
1998-Dixielandings 2002-CBR 2003-Offsite
2003-ASMo 2004-Offsite 2004-WL
2005-POFQ 2006-POFQ 2008-POP 2009-POP Sept. 2011-POP