Where/how can I learn U.S. Military tactics without actually joining the military?

Essentially - you can't.

The entire purpose of that training is to train soldiers to work as a team - individual marksmanship or individual skills are fairly easy to learn outside of a military setting, but they're world's away from the tactical teamwork that sets professional militaries apart from militias. The world's best marksman can be the world's worst soldier - maybe he has no situational awareness whatsoever, or refuses to take orders or suggestions, or some other reason.

What good is knowing how to flank a position going to do you if you have no one to work with, and what's the point anyway - are you expecting to need to assault a police station or something? Tactical military training is essentially useless knowledge for the average civilian. Technical training can be more useful, but why learn how a military mechanic works when you can learn how a civilian mechanic works?

Well, you wouldn't really have any reason to learn military tactics if you can't join the military. Even large police departments and SWAT don't use military tactics, they have their own "play book." If you just want to do something adventurous you could search for a local paint ball league. Even though it's just a game, they do develop some techniques and skills for that sport. You could join the US Coast Guard Auxiliary if you're near a body of water. The USCG Aux has a lot of retired military guys, though it's not like you'll be kicking down doors and requesting artillery fire in the Auxiliary. Most states have a State Defense Force as well. This is a legitimate (not anti-government) militia that trains for disasters, invasions, etc. Like the USCG Aux they have some former military members as well. These two groups don't have the strict medical requirements that the military does.

Probably the closest thing to what you want would be a private security company with overseas contracts.

Most of them won't hire you unless you are ex-military though.

You may learn at the location with these coordinates: 35°09′33″N 98°26′32″W

(you will have to copy and paste the coordinates into your browser bar)

Vietnam Infantry tactics.A book written by Osprey publishing.

You could do tactical rifle/pistol/shot gun courses. Some are taught by former military. That maybe the closet thing you can do as a civilian though.

Or become a police officer and when you have enough time apply for SWAT/ERT.

You are just as good as they are?? No you are not! You have not gone through the training... People in the training for SOFs and active members would kick the crap out of you for saying something like that. They earned their spot. You did not.

You did not qualify...deal with it.

NOT POSSIBLE!

No jobs unless you are ex-military!

check out youtube videos of military training