I would highly recommend apprenticing as well. I've been a groomer for 6 years. It's impossible to be a good groomer without ALOT of hands-on experience. It's amazing how many people don't actually know how to bathe a dog properly, or know how to tell if a dog is dry, or know which way to blowdry a dog's hair. You need to learn how to use your scissors as an instrument (you'll never use scissors the same way again). You need to learn ways to hold the dogs properly. What's safe and what's dangerous, etc. An online course just can't tell you anything about grooming. It's something you need to feel and experience (and it's most certainly not for everyone).
As for certification, the only way to TECHNICALLY be certified is to perform an exam in front of a panel of certified judges (You groom 2 or 3 dogs for them, I can't remember the details). Most groomers will call themselves "certified" after taking a course, but that isn't technically the case. They are certified from a course, but not certified groomers. Really becoming certified is pointless, it's easy, it doesn't prove you're a good groomer, it just makes you sound good, but you can just as easily lie and say you're certified so I don't see the point.
I'd just call different groomers in your area and see if any offer a grooming course or apprenticeship.