Check out AFAA and ACE'e web sites, they offer training all over the country. Also, the YMCA offers their own 'in house' training.
My advice would be to approach the Aerobics Coordinator at your club and let her know you're interested in teaching. Depending on the need for instructors they may offer to train you there or set you up with a mentor. Do you know any instructors that would be willing to mentor you?
Members/participants are very fitness savvy these days, they are educated, experienced and demanding at times. You may want to become an instructor and not spend a ton of time or money but I think you might be setting yourself up for failure. It's not as easy as it looks and there are many safety issues that need to be learned before teaching to the general population. One of the most difficult thing to learn is modifications. Modifications for every level of participant that's in your class, modifications for someone with a bad back, bursitis in their shoulder, bad knees etc. Also, intensity modifications...you'll have some that want to work harder/easier than others and you'll have to have options for everyone.
I don't mean to discourage you, not at all. We need instructors all the time but we need educated instructors who have spent the time and unfortunately some $$ to learn the trade. I'm an instructor, PFT and have run an aerobics program for 2 years and I'm still learning, it never ends...
Best of luck,
physfitgal