Here is a site with 2003 and 2004 estimated attendance numbers from Amusement Business Magazine:
http://www.coastergrotto.com/theme-park-attendance.jsp. If you google theme park attendance and the specific year you are looking for, you might be able to find more. Amusement Business will release their estimated attendance for theme/amusement parks for 2005 at the end of December. In the meantime, here are the latest released 2004 numbers
2004 numbers
USA Park Attendance
Most visited theme parks in North America
Theme park attendance figures are for the year 2004 and are represented in millions. Positive and negative numbers represent an increase or decrease in park attendance from the previous year.
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1 Magic Kingdom
- Orlando, Florida 15.2m
(+8%)
2 Disneyland
- Anaheim, California 13.4m
(+5%)
3 Epcot
- Orlando, Florida 9.4m
(+9%)
4 Disney-MGM Studios
- Orlando, Florida 8.3m
(+5%)
5 Disney's Animal Kingdom
- Orlando, Florida 7.8m
(+7%)
6 Universal Studios Florida
- Orlando, Florida 6.7m
(+14%)
7 Islands of Adventure
- Orlando, Florida 6.3m
(+13%)
8 Disney's California Adventure
- Anaheim, California 5.6m
(+6%)
9 SeaWorld Orlando
- Orlando, Florida 5.6m
(+4%)
10 Universal Studios Hollywood
- Universal City, California 5.0m
(+8%)
11 Adventuredome
- Las Vegas, Nevada 4.4m
(+2%)
12 Busch Gardens Tampa
- Tampa, Florida 4.1m
(-5%)
13 SeaWorld San Diego
- San Diego, California 4.0m
(0%)
14 Knott's Berry Farm
- Buena Park, California 3.6m
(+3%)
15 Paramount's Kings Island
- Kings Mill, Ohio 3.5m
(+7%)
16 Paramount Canada's Wonderland
- Vaughan, Ontario 3.4m
(+30%)
17 Cedar Point
- Sandusky, Ohio 3.2m
(-4%)
18 Morey's Piers
- Wildwood, New Jersey 3.1m
(-4%)
19 Santa Cruz Beach Boardwalk
- Santa Cruz, California 3.0m
(0%)
20 Six Flags Great Adventure
- Jackson, New Jersey 2.8m
(0%)
21 Six Flags Magic Mountain
- Valencia, California 2.7m
(-2%)
22 Camp Snoopy (Mall of America)
- Bloomington, Minnesota 2.6m
(+5%)
23 Hersheypark
- Hershey, Pennsylvania 2.5m
(-2%)
24 Busch Gardens Williamsburg
- Williamsburg, Virginia 2.4m
(0%)
25 Six Flags Great America
- Gurnee, Illinois 2.3m
(0%)
37 Dorney Park
- Allentown, Pennsylvania 1.4m
(+1%)
44 Knoebels
- Elysburg, Pennsylvania 1.3m
(+3%)
Source: Amusement Business