Is there any official DVD or ISO of all Amiga modules?
Where can I buy it?

You used to be able to buy the Mods Anthology set (4 CDs) which went up to the 1995-1996 era or so, it's probably online somewhere and has a bunch of stuff direct from the composers.   I don't think there's been another attempt to do something so vast since because it's a ton of work and everything is pretty much online these days.  (modarchive,amp etc.)

I am guessing the best source today is modland at which still seems to be maintained.

yeah modland seems to have almost everything I've ever wanted (ie. everything fleshbrain ever released). The Modizer app is a good way to download from modland and play it on your phone, too, if you're that way inclined!