What can we do to get just one single grocery store accepting monero?

Put yourself in the place of the grocer. If one person asked you to accept platinum as payment, would you? Probably no. If 10 people or a few loyal customers did, you might look into it.

Get prepaid cards from online stores against monero, that's been my solution for any discrete purchases I'd like to make.

I think the real problem would be that it would have to be in a place that accepts Monero more broadly. Like, for example, if someone started a grocery store, then they can't really ship things to you. So they need to have a large enough community locally to sustain that.

A better question is to ask how do we get a single farmer accepting and requesting monero usage.

The grocery store is a middle man that does little creation of value except occupying a location and marking up products to pay for electricity and labor, all of which must be paid in fiat by law.

Many farmers eek out a living off grid, and without any labor costs.

Given grocery stores tend to use the currency corresponding to their government's authorized legal tender for use in payment of debts: a lot of small things. A select number of stores in different regions accept bitcoin one way or another, word-of-mouth to support purchasing with Monero through similar systems can encourage practical consideration of it. It will take the slow expansion of Bitcoin-accepting businesses to also accept Monero as value that can benefit the business. In general, I don't know of any specific grocery store in my region that accepts crypto, Monero has an uphill battle to see grocery deployment.

At this point the owner would have to be into Monero for it to happen.