You’ll have to check with an authorized dealer to see if there is still inventory available at Titleist. There may still some outstanding 2019 T100 orders.
If not, then a 3 iron single on an auction site like eBay or resellers like 2nd Swing or 3Balls It may require obtaining a club and reshafting. Although obtaining a single iron shaft is difficult from most golf shops as they tend to only supply full sets of shafts.