NRA LE Division has some pretty loose requirements for admission (Active LE or Mil, retired LE, and some security). I've done their Handgun/Shotgun course.

FLETC has been mentioned. Probably the best bang for the buck for LE...everything paid for by the feds - class, ammo, food, lodging.

Sig Academy offers a 40-hour course, and several follow-on courses, open to civilians.

S&W Academy used to offer an 80-hour Instructor course. Cost included ammo.

USCCA and SAFTD offer Instructor ratings.

Tom Givens at Rangemaster offers a 3-day Instructor, 2-day Advanced Instructor, and 3-day Shotgun Instructor course.

FBI and DOE both offer active LE 80-hour courses.

Pat Mcnamara, Aaron Cowan at Sage Dynamics, Thunder Ranch, Gunsite, and Paul Howe at CSAT all offer Instructor courses.