Where can I find Jumbo Sun/Star/Moon Drops?

If i need them i just make them, that's alot easier then grinding for rare drops unless you don't mind grinding. The small ones you can steal from the enemies where your at around that town.

Jumbo Drops can drop or be stolen from enemies rarely (up to Nevermore castle and then some), but as others have stated the easiest is to make them for yourself.

If you look in the Wizard's Compendium under Alchemy there should be a recipe for the Jumbo drops fairly early on, however, failing that it is not listed, it is as easy as combining one of the regular drops and its corresponding celestial crystal. For example a Jumbo Stardrop is created with the use of one regular Stardrop and one Star Crystal.

Actually, in order to alchemize jumbo drops, you need one drop and *three* of the corresponding crystal types--not just one. Using the incorrect number will result in failure. These are the recipes:

1 Sundrop + 3 Sun Crystals = 1 Jumbo Sundrop
1 Moondrop + 3 Moon Crystals = 1 Jumbo Moondrop
1 Stardrop + 3 Star Crystals = 1 Jumbo Stardrop
1 Planetdrop + 3 Planet Crystals = 1 Jumbo Planetdrop

you can also purchase them with chips in the casino in the tombstone trail. Its is not cheap though.

As previously mentioned by usagi1288 you can buy them in the casino..
You gain access to it essentially after the Tombstone Trail section, I'm sure you can work out where it is...

However, you can't buy the jumbo drops until you complete another section of the main quest..
Don't wish to spoil so I won't mention when specifically, but let's say it's possible to buy them once you can visit a certain island once shrouded in mist.

You can easily get tonnes of of chips from the casino at the platoon table, with a small bit of luck and some knowledge of the npc. If you play it right you can turn the 200 chips you first place in... Into over 500k chips, which given the jumbo drops are 3k each is more than sufficient. (200 chips is 1000 guilders btw).

Otherwise you're looking at defeating enemies for crystals, but also using the one and only foraging location that gives crystals, which is in a place where you can find Mitey Mites