You definitely need a naga, but if you try to farm it outside of Scarlet Snake Princess... well, have fun with that. I tried all the normal dungeon drop spots and went 0 for more than I can count before SSP finally came around. Unfortunately you just missed a SSP last week.
Other good mobs to consider -- for green, get your Puzzdroid from the Mysterious Visitors special and turn it into a Top Droidragon. For red, pick the correct egg that turns into a red snow globe dragon (it's the one that says Happy Holidays during the fight, if you get something else exit the dungeon and try again). Both of those are easy to evolve if you devote a Thursday to doing the plant dungeon. Dump ~250k xp into the snow globe dragon and you've got a good sub for now, and 1.5M worth of xp fodder for Leilan down the road a bit. For filler, you can use other dragons (rippers from the Pal machine, starter dragon, rank 20 present if you happen to get a useful one, etc.), dub-lits, and keepers.
For friends, you'll want to find a whole bunch of people who have Leilan, Bastet, or Kushinada up all the time. Possibly Sakuya and Horus as well, since Top Droidragon will trigger Sakuya's blue/Horus' 4th color requirement.
I would advise making a resolve team right away too. Solomon Grundy from the Batman collab going on now + Top Droidragon + 3 sapphlits + friend evolved Siren = trivial clears of the expert tier wed/fri dungeons. Alternatively, go green and use ogre + copperhead + 3 emelits + evolved Alraune. That will get you more dub-lits as well as masks for your evolves. Masks are better than spirits for your wed/fri clears too.