OK this is a half serious thread but in 8 seasons I have never had one drop.
Are you guys getting yours from boss runs or somewhere else? If I pick a lesser boss chest at Belial does that mean I have less of a shot for a mythic then choosing durial or Andarial?
Inquiring minds want to know.
i got mine by crafting 2 sparks from spectral ash scrolls.
farming lesser bosses is hopeless, the devs heavily nerfed them this round. in the previous season, i already had 2–3 mythics from EACH lesser bosses by week 3, but this round, 0.
I don’t know anything about s8. But I’ve found dozens in previous seasons by spamming bosses and undercity with the unique tribute.
Gambling for a mythic this season.
Submit yourself to Blizzard’s unspoken demands and use diablo trade to sell the good ga uniques you get and buy ohms. Also you can level alts rep with the new scrolls to get more sparks. That or suffer.
PS. in order to spare you some pain - get the gyazo free program to snip an area of the screen. You will need this to make a listing, I think, not sure if you can do it 100% manual.
Crafted. Played, saved, farmed, learned lilith and a bit of buying/trading in chat.
my first 3 drop was all selig, angry and salvage, then I crafted a granpapa.
the very next day it drop me another grandpapa.
then yesterday It finally drop 2 starless skies.
The trick is Ive change from Barb to rogue. If you wanna a starless, played rogue. coz it has the less possible item in the pool of uber.
I’ve got mine from belial.
First Shako I crafted at the jeweler this season, had a GA on cooldown. Right where I needed it, I’ve been good on mythic hunting since 
Duriel and then Undercity with an Ascendant tribute for the two I’ve got so far.
I had 8 Mythics drops but no starless skies. I used none of the Mythics in my builds this go around, there was no need. They will get salvaged as usual when they come over to the Eternal Realm.
I ordered it from a Bijou Brigitte catalogue i found in the knitting basket of my mom. 
Ok… i got 3 ones so far. I got 2 of them from the small 1-shot bosses (Grigoire & Varshan). One i got from Duriel.
I crafted the first one I had on my barb at the begining of the season. Few dropped on bosses but then crafted all the rest.
classic confirmation bias. Sometimes you get unlucky. sometimes you get lucky. Now, isnt it great that no mythic unique is build defining?
god, sounds like my last seasons curse went to you for s8 =P
Update:
In a truly, truly, bizarre turn of events after I made this thread I farmed duriel and Belial last night. Upon choosing Uriel’s chest for rewards, not one, but 2 starless skies dropped. One GA and one 2GA. I was absolutely stunned.
Did someone at blizzard read my post and then turn the spigot on it something? It’s just weird.
In 8 seasons I’ve never had a million dollars of real money. Heck it’s worth a shot!! Lol.
My first and only from a 2 spark cache.
lucky 2 GA from duriel
In previous seasons for me Duriel was my main Mythics source including that ring. I can say 80% of the drops were from him. The other 20% were from other sources like pit, undercity, runes/sparks, etc.
So far this season I have not found any starless ring yet.
I got my Shako from a lucky blacksmith cache with my first 2 sparks. GA cooldown as well which is all I cared about for my incinerate sorc. Triple hit that baby for 54%… then got my Starless from my first T1 Andariel kill. GA on +core stats which is all I needed as well… attack speed and crit are useless for my build. Triple hit that to +7 and I’m officially done needing mythics this season.
Belial. 20 chars wooo