Availability And Timeline
Patlabor was first released as a direct to video series or "Original Animated Video (OAV)" by the animation group Headgear in 1988-1989. The series became so popular that a movie was later released (Patlabor the Movie; 1989) as well as a 48-episode TV series (1989-1990). Another 16 episode OAV was later released (Re-titled The New Files by US Manga Corp.; 1990-1992) as well as a second movie (Patlabor2; 1993). A new movie (XIII Patlabor) is was released in 1998 and is loosely based upon one of Masami Yuki's Manga Stories.
However the actual chronological order of the series (ignoring the original pilot) goes like this. First the TV Series from episodes 1-25. Then the first movie, The remaining episodes, the OAV, Patlabor XIII. And finally Patlabor 2. Also note that pilot (original OAV) episodes 3-7 can be added anywhere in the first 23 episodes of the TV Series with out causing too much confusion.
Availibility:
Patlabor the Mobile Police: The Original Series OAV (aka Kido Keisatsu Patlabor OAV) available from US Manga Corps
Patlabor The Movie (aka Kido Keisatsu Patlabor 1) available from Manga Entertainment
Patlabor the Movie 2: Patlabor2 (aka Kido Keisatsu Patlabor 2) available from Manga Entertainment
Patlabor: The New Files (aka Kido Keisatsu Patlabor) available from US Manga Corps
Patlabor: The Mobile Police (aka Kido Keisatsu Patlabor: On TV) Available from US Manga Corps
XIII Patlabor (aka Waste Product No. 13) Availability Unknown
All of the above titles are videos unless otherwise noted. All prices were taken from a copilation of catalogs and are presented FYI they are not necesarily the suggested retail prices as I am sure that you can find them cheaper.