Hm... are there lemon trees? Not sure how to find the potatoes - are there little divots in the ground with them in the fields?
Pretty sure you can buy them at Herbalists like the one in Silent Hills. Though I could be mistaken, been a while since I played a techie.
You cannot buy them.... at least I have yet to see one appear and I often advance time by a day and search for supplies I need.
It's been a while since I went looking but here's what I remember for certain: Spoiler Lemons can be obtained from a tree at Black Root and the herbalists in Tarant and Caladon. Potatos are in a plant or two in Shrouded Hills, herbalists in Tarant and Caladon, and appear during random encounters - rare, but I have seen it.
An herbalist does sell both. You just have to wait it out. You rarely find potatoes out in Arcanum so it doesn't come in handy too often. There is another way to make batteries that is a lot easier, or just buy them. There are lemon trees. The herbalist in Tarant will sell them also you might try the Wheel Clan. They have a little garden area I am not sure of what is in there, they have an herbalist too so you might try there also.
Herbalists don't sell potatoes nor lemons in vanilla Arcanum, of that I am certain. Drog could add them to their inventory in the UAP - like he added various barbarian clothes to the stock of rural smiths - but from what I've experienced, he didn't. I do recall seeing potatoes and lemons in shops in Vormantown and I don't know what is their availability in A:WIP or other mods, but those situations are a whole different story. From what I know, the few unrestockable lemon trees and potato plants are the only sources of potatoes and lemons in vanilla and vanilla+UAP Arcanum. Some common plants and a few extremely rare ones as well, but no potato plants nor lemon trees.
Yes, only the Vormantown merchants sell potatoes and lemons (they have different, MP oriented item lists).