Where to purchase SIM card.

I want to make a sump pump alarm that will send me a text if my pump fails. Is there a SIM card I could purchase that I could prepay for X number of texts with no voice plan or monthly fee?

Where are you?
That will help people suggest options.

I'm in rural Kansas. Verizon, US Cellular and Straight Talk work best in my area. There may be others but those are the ones I'm familiar with. I tried asking Straight Talk and Verizon, but they really didn't understand what I was asking g.

I sell SMS controllers, and I’ve found the best deals are usually prepaid plans with unlimited text (minimal data).
If you can find one that is compatible withyour modem (bands and modulation schemes), you’ll be on the way.

So, if I bought a prepaid plan for the minimum minutes possible with unlimited text would it still be good in, say 5 years, if I didn't use any minutes?

as long as you maintain the plan, usually a small amount each month.
Here in Australia that’s about $10, or USD 6 per month for unlimited calls & text, with a tiny bit of data if you need it.

$6 a month is more than I want to spend for something that may only be utilized once every couple of years if that much.

Then you may need to look into another approach.

If it’s a single site, perhaps you can sign up a second SIM card on your personal phone plan…