Find a replacement by soldering handpiece
Start with the model printed on the handpiece. If the station was bought used or the iron was replaced, the station name may describe the power unit and stand while the handpiece determines which tip seats correctly.
Look up the handpiece in the manufacturer's tip chart and record the full part number before comparing shapes. An FX-8801 example uses T18, while Hakko lists T15 cartridges for an FX-951 setup with a compatible pencil. A chisel width such as 1.6 mm is a dimension shared across families.
If a listing names both a station and a part family, confirm the exact suffix. Weller's WEP70 chart calls out exceptions within the ET family. If your old tip is ETS and the pencil is WEP70, compare the exact Weller reference and revision before buying. If your pencil is PES51, consult the older WES51 parts list.