Craigslist - a very cool and free site - no ads either.  Its a great resource for many things.  If you feel adventurous just go there and poke around yourself to see what is listed.  If you are in a hurry then try clicking on some of these predefined searches based on cities.  Since the searches are for reality base television shows you will find the majority of the casting notices will be posted on the LA portion of the list.  But some of the other cities do have information about casting calls in their areas.  Just an FYI if you do not see an address listed for contact then you need to mail to the address with @craigslist.org and it will be relayed to the person that posted the ad.

The searches are simply looking for anything with the word  "casting" somewhere in the posts in the category of "tv/film/video/radio jobs" so you will find non-reality casting posts as well.
 

A
Atlanta	Austin
B
Baltimore	Boston
C
Chicago	Cleveland
D
Dallas	Denver	Detroit
E,F,G,H
Honolulu	Houston
I,J,K,L
Las Vegas	London, UK	Los Angeles*
M
Miami	Minneapolis	Montreal
N
Nashville	New Orleans	New York*
O,P
Philadelphia	Phoenix	Pittsburgh	Portland	Providence
Q,R
Raleigh
S
Sacramento	San Diego	Seattle	St Louis
T
Tampa Bay	Toronto
U,V
Vancouver
W
Wash, DC
X,Y,Z

*These cities have A LOT of casting calls so I have added the extra word "reality" to the search to eliminate the general casting calls for films/plays/etc.  You can simply follow the link and remove the word "reality" from the search box and re-submit the search to get all casting calls.