You can see for yourself here:
http://173.255.246.196/2016/
Do remember that there are generally more slots than shown that get served out to waitlisted teams, late registering rookies, etc.
Regionals with general admission slots available:
Dates -------------------- Event Name ---------Filled - Left - Total
09-Mar - 12-Mar -- Arkansas Rock City Regional -- 43 ----- 9 ---- 52
16-Mar - 19-Mar -- Utah Regional ----------------- 37 ---- 12 ---- 49
23-Mar - 26-Mar -- North Bay Regional ------------ 30 ----- 8 ---- 38
30-Mar - 02-Apr -- Idaho Regional ---------------- 22 ---- 13 ---- 35
31-Mar - 03-Apr -- Hub City Regional ------------- 40 ----- 3 ---- 43
03-Apr - 06-Apr -- Western Canada Regional ----- 33 ----- 5 ---- 38
Districts showing open spots:
Dates -------------------- Event Name -------------------------------------------------- Filled - Left - Total
18-Mar - 20-Mar -- CHS District - Northern Maryland Event ---------------------------------- 32 ----- 2 ---- 34
17-Mar - 19-Mar -- PCH District - Albany Event ----------------------------------------------- 18 -- 14 -- 32
17-Mar - 19-Mar -- PCH District - Dalton Event ------------------------------------------------ 29 -- 3 -- 32
18-Mar - 20-Mar -- MAR District - Springside Chestnut Hill Event ---------------------------- 27 -- 2 -- 29
18-Mar - 20-Mar -- NE District - UMass-Dartmouth Event ------------------------------------ 30 -- 1 -- 31
24-Mar - 26-Mar -- NE District - Rhode Island Event ------------------------------------------ 33 -- 2 -- 35
18-Mar - 20-Mar -- NC District - UNC Asheville Event ----------------------------------------- 21 -- 15 -- 36
01-Apr - 03-Apr -- NC District - Campbell University/Johnston Community College Event -- 31 -- 1 -- 32
03-Mar - 05-Mar -- PNW District - West Valley Event ------------------------------------------ 34 -- 2 -- 36
24-Mar - 26-Mar -- PNW District - Philomath Event ------------------------------------------- 28 -- 8 -- 36