Good luck getting just that one.
We’ve had 5 ‘tetanus boosters’ in my family in the past 6 or 8 years and every single one was the multi TDaP. ER doc told us the single tetanus only was nearly impossible to get anymore.
Walmart, May 2017. In and out in a few minutes. Tetanus-only booster. Maybe the ER doesn't have the tetanus-only version so they can charge you more. Of course, mine was a necessity for being a troop leader in Trail Life. Well, that and it was time for a booster, since it had been more than 10 years since my prior booster.
My PCP has them. Got one a few months ago.
Probably ER stocks the trivalent to immunize the herd. Lots of decisions are made that way, like the hep shots for babies. Immunize before they get away rather than when they might need it.