There are no direct flights from HK to Yiwu. I suggest you goto Shenzhen airport from HK airport and then take the domestic flight to go to Yiwu. You know domestic flight tickets are usually much cheaper than the international ones. If you do not mind booking on the site, I may recommend a very good site for you:

https://www.travelchinaguide.com/china-flights/

It can help you book cheap ticket. It's very convenient to use the e-ticket. You do not need to get the ticket in person. When you go to the airport, just show your passport, visa, ID card and the like, you can board the airplane.

hi ,mr naru,
there are several way from hongkong to yiwu:
1, go to guangzhou first ,then go to yiwu by air from guanzhou.
2, from hongkong to shanghai by air ,then from shanghai to yiwu by train.
3, from hongkong to yiwu by directly bus.

I think these 3 ways are good for you .

have a good trip .