This has been answered pretty thoroughly already but I’d like to chime in because I cook a lot!
The best place to get it is from the Cooking Camp, its quite infrequent (i think a week every two months or so) and gets more use when you’re an older player and can cook faster + have more slots for cooking so you can get vouchers easier, but still definitely obtainable for lower levels. But it’s the only place you can get salt in bulk in a reasonable fashion.
The mole market is the second best place, because it requires no work other than being at the right place at the right time! That being said, it’s a bit of a pain to rely on that since the mole market refreshes just enough/not enough to be inconvenient, but you can get 10 for a very good price all things considered. The most beginner friendly!
The goldsea finding ingredients….is kinda bad. I personally do not like it since it gives you about two drops and sometimes it’s salt. That’s on top of getting that option in the first place, especially with the other explore options in that area being a bit lackluster as its in the first area. I mean for crying out loud, the fishing option gives you six fish! The many cons aside, its permanent and repeatable, so if you’re in a pinch its a worthwhile option.
I don’t recommend getting salt in the flea market. Salt is highly in demand for its difficulty to obtain and the prices I’ve seen are atrocious. Be ready to pay ten times as much for one salt.
And technically, ingredients aren’t locked behind levels, by the sense that its not that you can’t use them at a lower level like how it is with mining equipment or crafting recipes. Level just affects methods of obtaining and the success rate of your cooking. If a concern is you’ll burn everything you make since it’s too high level, then you can at least collect ingredients so you can use them later!