Also cavs.flashseats.com is by far your best bet for tickets. Single tickets tend to be a little cheaper than pairs. Not sure how much they will run- so far throughout the playoffs for upper level tickets they were around $50/ticket for the Boston series, $60-70 for Chicago, and around $90 for Atlanta. For the Chicago series game 2, I bid on some lower level row 18 tickets the day of the game around 4pm and got them for $80/ticket which was listed as face value on my invoice. They tend to get cheaper the day of, and there are generally a decent amount of tickets so even if you're coming from out of town you shouldn't have to worry.
No clue about celebrations after since we don't have much experience with that...