Water Lily, Kelp, and Shells, Oh My! There is only 1 creature that I am aware of that drops all 3 of these reagents and that is - oh um Rattlebones KINDA. Oops, I didn't read that very well before starting this post. Rattlebones (Rank 7) in the Rattlebones Master Challenge drops Water Lily and Shell while it is Rattlebones (Rank 14) in the Rattlebones Exalted Challenge that drops Shell and Kelp. Ok, my bad. It doesn't look like anyone drops all 3 reagents. There could be, I don't have the scoop on ALL the bosses out there yet.
The Grand Tourney Arena just might be your best choice if you are need of all three of these reagents. They are all dropped by one boss or another. Bad news: Not necessarily all in the same tier. I'll poke into that deeper in a minute.
The only critter that drops Water Lily, that is NOT in a housing instance, is the Enthralled Spirit Monk in the Pagoda of Harmony in Kembaalung Village. That is about the only interesting thing that he drops, he isn't even on my radar for information to gather at this point.
Any tier of Tanglewood Terror will have a chance of producing Water Lily. All Dead Beets, Desparagus (tiers 2, 4, & 5), Meanest Fly Trap (tiers 1, 2, & 5) and Forest Grump (tiers 2-5) claim to drop it. You can only get this housing instance with crowns of course but it is your best bet for Water Lily.
You also could possibly grab it from Meowiarty & Krokopatra in their Exalted Challenges.
You can Click this link here to see the 4 people I have listed that drop both Kelp and Shells, but I'm pretty much going to be covering them anyhow.
In the Grand Tourney Arena you might collect Kelp from Lady Tanselle DreamingStar (Tiers 1 & 3 - 5), Sir Dunc (Tiers 1 & 3 - 5), Sir Lyonel LaughingStorm (Tiers 3 - 5), and All tiers of Sir Steffan BadApple.
Other vermin that you might be able to con out of Kelp include Meowiarty in his Archmage Challenge; Krokopatra's Exalted Challenge; Angler Thugs, Dr. Zigmund, Riverclaw Guards, Royal Guards, Telluric Eels, and Tower Captain all of which you'll find in various locations of Crab Alley. Surprisingly Nalia Dunestrider in Wysteria might also furnish it if you sweet talk her. There are a couple of other people scattered throughout the Spiral if you feel like running around all day. I wouldn't personally.
There are 125 different things that could net you Shells. I have a list of 67 of them in my knowledge base.
Lady Tanselle DreamingStar (Tiers 3 - 5), Sir Lyonel LaughingStorm (Tiers 3 - 5), and Sir Steffan BadApple (Tiers 1 & 3-5) have been reported to drop Shell in the Grand Tourney Arena.
As far as the others, you really want to narrow it down by what else you might be farming for. With the new pet tome, if you are hunting for pets you could be missing, here is a list of 19 varmints that also drop assorted pets. I also have a list of 10 critters that drop Kelp and Mega Snacks. Oh look, Sir Steffan BadApple (Tier 3) is on that list along with both Housing Instance versions of Rattlebones. That's about all I got, none of them seem to drop Skeleton Keys, Housing Instance Challenges, or the Phoenix Treasure Card if anyone is still hunting for it.