With the exception of the Genuine E3D v6 & the Genuine E3D Lite6 the replacement hotends listed below are not necessarily recommended replacement options, i.e., they will theoretically work but the quality may vary. These are not the only options available. These should give you a good idea of what to look for. Please do your research or ask the community about their experience.
Some of the options listed below will require updating the firmware's thermistor value using the custom G-code command "M561 P1". If you do change the thermistor type/value you will need to run PID Autotune. If you are able to reuse the Select Mini's original thermistor it is STILL recommended to run PID Autotune
E3D-Online - Genuine E3D V6 All-Metal HotEnd
Printed Solid - E3D V6 HOT END KIT
Filastruder - ALL-METAL E3D V6 HOTEND
E3D-Online - Genuine E3D Lite6 HotEnd
Printed Solid - E3D LITE6 BUDGET HOT END KIT
Filastruder - E3D LITE6
Amazon.com - Metal J-head V6 Hotend 1.75mm Filament 0.4mm Bowden Extruder Reprap 3D Printer with 1 Meter Tubing
NOTE: Does not require updating the firmware's thermistor value.
Direct Voltage - Metal J-head Hot end for 1.75mm. Extruder. with Fan, Heater & Thermistor 0.2, 0.3, 0.4, 0.5 OR 0.8mm nozzle
NOTE: You must choose which nozzle size. The Select Mini comes with 0.4mm. Does not require updating the firmware's thermistor value.