I bought all the things for my axie aquarium separately. Mostly because, like you said, there's no "kit" for 20 longs that I could find. You also get to pick the filter out personally and can get a better quality one than is available in most kits. I set my aquarium up two years ago and I'm still running most of the same equipment. For me, it was more useful to hand pick all of the items, since axies aren't fish and their equipment is varied slightly.
I did get a "kit" for my ten gallon betta aquarium. It wasn't sold as one, but the tank had a coupon book for all the things that go in a tank, so I just used that. A little lower quality, but my betta doesn't seem to mind.