Hi Alpharazero,
Some commercial juices (mostly, I know the Juul uses benzoic acid). As far as juices like Mr. Salt E, I'm unsure. You could certainly email the vendor and ask.
I DIY with nic salts, and I use Nicotine River, I believe they use citric acid (although my memory may be failing me there, but its' NOT benzoic acid.
Really, any DIY wholesaler might be willing to tell you if you ask. You can also get samples from various vendors (DIY and/or manufactured
juice) samples to try often very inexpensively or for the cost of shipping. There are also folks converting their freebase to salt, using citric acid crystals, but that gets a bit more involved. Although, you would have "total control" over what you use to convert, which can be nice. I think the big three are benzoic, malic or citric acid.
Best of luck,
Anna