In my opinion the best training is to get work in PV with someone who is intelligent and knowledgeable and values training their employees. :thumbsup: But of course if you're not so lucky to find that...
I would recommend looking at Solar Pro Magazine's training calendar. You may have to sign up for an online subscription, but reading their articles would be good training, too, probably at less cost than courses. And yes, if a course has NABCEP sponsorship or endorsement or credits, that is a good sign.
Be aware of the difference between the NABCEP entry level course and full NABCEP certification. It's relatively easy to take a short course and pass the entry level exam, and if you have no experience it's a good thing to have on your resume when you start looking for work in PV. But it's not the same as being a 'NABCEP Certified Installer'. The latter generally requires years of installation experience and/or engineering degrees to qualify for the exam, and is a harder exam to pass.