Solely, just, exclusively, simply...

EG: "Mrs. Smith solely/just/exclusively/simply buys oranges at Bob's Market."

I got my ideas here: https://www.merriam-webster.com/dictionary/solely

Or, "Mrs. Smith never buys anything but oranges at Bob's Market."

What about "single-item shopper" or using the item in question as the descriptor/adjective?

"Apple-seeking shopper hits pay dirt at Bob's Market."

"Apple-seeker finds missing ingredient at Bob's Market."

"Apples-only shopper, Mrs. Smith, pleased by Bob's bushels of Braeburns."

Sounds goofy, I guess.

You could also say something like, "Shopper on an apple mission/quest/search, Mrs. Smith, found what she needed at Bob's Market." It doesn't necessarily mean that she didn't buy anything else, but it's pretty clear that she did find the one item she wanted: apples.