the built in ones are https://jossef.github.io/material-design-icons-iconfont/
You can also you iconify icons that then gets them from the web - eg
[{"id":"f011384e.a5c978","type":"ui_button","z":"e9b4dddd.70115","name":"","group":"463fb27.9c2064c","order":3,"width":0,"height":0,"passthru":false,"label":"Garage","tooltip":"","color":"","bgcolor":"","icon":"iconify-mdi:garage-variant","payload":"","payloadType":"str","topic":"","x":360,"y":480,"wires":[[]]},{"id":"91c1c10a.8715","type":"ui_template","z":"e9b4dddd.70115","group":"caea69a6.a956a8","name":"","order":4,"width":0,"height":0,"format":"<script src=\"https://code.iconify.design/1/1.0.7/iconify.min.js\"></script>","storeOutMessages":true,"fwdInMessages":true,"resendOnRefresh":true,"templateScope":"global","x":180,"y":480,"wires":[[]]},{"id":"463fb27.9c2064c","type":"ui_group","name":"test","tab":"cc2c465a.11e6d","order":1,"disp":true,"width":"8","collapse":false},{"id":"caea69a6.a956a8","type":"ui_group","name":"camAlerte","tab":"250de740.57ee78","order":10,"disp":true,"width":"6","collapse":true},{"id":"cc2c465a.11e6d","type":"ui_tab","name":"test","icon":"dashboard","order":1,"disabled":false,"hidden":false},{"id":"250de740.57ee78","type":"ui_tab","name":"Dashboard","icon":"dashboard","order":1,"disabled":false,"hidden":false}]
gives
but note this will only load them from online so your browser needs to see the web at all times.
See https://iconify.design/ for more.