On the Available Gathering Achievements page it says for Wopal Shell harvest it says

The dish "Underground Shafts Conch Salad" increases your chances of harvesting miscellaneous items

I do not see the salad as an option in the Culinary Shop is this in the game and if so where is it?

That salad is not available at this time since it requires a Lobster from a currently unreleased Expedition. The Wopal Shell itself can be harvested in any Wopal Expedition, and it is a Medium-sized item, so your best bet is to check for any Food that boosts Medium-sized items.

You can only get it from the facility areas I believe. (Including the Level-Up Quest variant.)

They need to remove the message from the game and replace it with one of the medium boost foods then if you can't get the salad yet. I can confirm Floating Facility Exploration has the shells just got 5 (2 and then 3) from 100 energy tried yesterday on Coast Exploration and spent 300 energy all during a fever and didn't get a single one

@Miarkraim They can also come from Coast and Seabed, as I have obtained them from both personally; it is honestly just RNJesus making things a pain. I once went through 5 characters, using the appropriate food, trying to get Harkotan Fragments, and I ended up with 3.

As the item description says, Wopal Shell can be harvested anywhere on Wopal. I just got a few from Seabed

As @AndrlCh said, we only have a choice to use medium-size boosting food now in NA. Ruins Shrimp Tempura (+100% medium-size) and Underground Shafts Mushroom Mix (+100% medium-size) are the dishes we currently have. Both are just 100% boost so might not have a great effect, unfortunately.

@Miarkraim If you are going to use medium-size booster, maybe you'd better avoid Seabed. Seabed has two other medium-sized harvesting items aside from Wopal Shell, so the chance you get Wopal Shell will be relatively lower. Coast and floating facility both has another 3 star medium-sized items, but that's better than seabed.

Is there a list or guide somewhere showing the best locations, and which food boosters to use for each 100K item procure contract?

@BLACK-x-SCVLE I'm not sure. I just made deductions from JP server data and crossed out items not available in NA.

Although I did write some detailed answer, I'm actually not a fan of using food boosters. Their poor performance often doesn't match the cost of preparing them. The only dish I often use is Extravagant Seafood Platter for jewels.