For my big boys this is my current favorite go to https://smile.amazon.com/gp/aw/d/B002H3N0IC?psc=1&ref=ppx_pop_mob_b_asin_title

For smaller ferrets, finding a good harness is a bit more challenging. If your ferret is a baby then I would wait a bit till they get bigger. If your ferret is an adult but small then I would try esty or something like these

https://smile.amazon.com/LXT-YY-Outdoor-Walking-Accessory/dp/B089QCD9DN/ref=mp_s_a_1_8?crid=2YNFNCRQ98ITL&keywords=ferret+harness&qid=1675028414&sprefix=ferret+harness+%2Caps%2C394&sr=8-8 or https://smile.amazon.com/Ferret-Harness-Adjustable-Walking-Breathable/dp/B07DKCKS6T/ref=mp_s_a_1_10?crid=2YNFNCRQ98ITL&keywords=ferret+harness&qid=1675028550&sprefix=ferret+harness+%2Caps%2C394&sr=8-10

Hope that helps