Are there plans that don't include it?

I have quite possibly the cheapest possible t-mobile plan (prepaid, $40/month) and it includes 4x full flight wifi and unlimited 1hr (1 per flight) wifi. It works on AA too, which it didn't used to when they only had the 1hr per flight thing. I fly a lot of short flights (PHX-LAX) so it's great because 1 hr covers the whole flight.

Even if I was with another carrier, it might be worthwhile to get a prepaid line just for this. Or if you have a friend with a t-mobile plan who doesn't fly, see if they are ok with you using theirs (you just need to enter the phone number; there's no validation done as far as I can tell)