how can I get XP after crashing Sarevok's party?

I defeated Sarevok's Doppleganger assassins at his party, and have decided I would like to have characters higher than levels 6-7 to go into the undercity. I have beat Ulcaster and beneath Firewine and do not have TOTSC, where can I go to get experience.

Well, how much of the city have you "investigated"? If you've seen and done everything, you might want to check to see if you've missed any areas out of the city--especially near the coast. Did you go "under" Ulcaster? Also, did you head into basilisk territory (near Ulcaster)? Aside from that, all I can recommend is going to the undercity anyway. You won't get much higher than 6 or 7th level anyway due to the exp cap, and you will meet plenty of interesting foes before you encounter Sarevok again, all providing large amounts of exp points (provided you can beat them). Be well rested, and don't be afraid to spend money on things (having 10,000 gp doesn't matter much if your favorite characters are dead)--especially potions (visit Sorcerers Sundries again?).