-By Plane
1) Fly directly from Hong Kong to Yiwu
Yes, there are flights from Hong Kong to Yiwu directly. But not every day, please kindly check the flight details first before you come.
2) Fly from Hong Kong to Hangzhou, then bus to Yiwu.
This is the fastest way. Usually several flights from Hong Kong to Hangzhou each day, and 10-20 buses from Hangzhou to Yiwu (1.5-2 hours, RMB 60.00). Bus departures from Hangzhou airport for Yiwu timetable:
3) 10:20, 11:00, 11:40, 12:20, 13:00, 13:40, 14:20, 15:00, 15:40, 16:20, 17:00, 17:40, 18:30, 19:30, 20:30, 21:30.
4) There are also plenty trains from Hangzhou to Yiwu, the fastest ones just take 55 minutes. But you need to go to Hangzhou Railway Station first (seems more effort, just put here as an option in case).
5) We can pick up you at Hangzhou Xiaoshan Airports. The price is 350RMB or 60USD per car. Please click Hangzhou to Yiwu.

-Train from Hong Kong to Guangzhou, and then fly to Yiwu.
This is ideal if you also have some business around Guangzhou.
From Hong Kong Airport, subway to Hung Hom subway station, cities express train to Guangzhou East Train Station. (RMB 190). This way costs less, pretty punctual, no traffic jams.
After Guangzhou business, 2 hours flight non-stop from Guangzhou to Yiwu. Usually there are 3-5 flights from Guangzhou to Yiwu each day, if you can book 2 weeks in advance, you can usually have a good discount up to 70% off (this rule works with all China domestic flights).
For more about how to go to Yiwu from Guangzhou, Please follow Guangzhou to Yiwu.

-Bus from Hong Kong to Shenzhen, and then fly to Yiwu.
This is also ideal if you have some business around Shenzhen.
From HK airport, simply take a bus to Shenzhen (at least 1-2 hours). This way is just easier, sometimes may have traffic jams. (The timetable below is JUST for reference).
And you can even take a boat, or train, or subway then transfer to city bus. All seems too much effort if you are not familiar, but the good thing is you can never be lost in HK as everyone can speak English.
After Shenzhen business, 2 hours flight directly to Yiwu. Usually 1-3 flights from Shenzhen to Yiwu each day. Just check for sure while scheduling.
For more about how to go to Yiwu from Shenzhen, please follow Shenzhen to Yiwu.