If you are traveling within China - Ctrip is the easiest one to use - for train and plane bookings.
Train from Guangzhou
You can take a train from Guangzhou to Yiwu.
It is 6.5 hours. Some feel this may be better than a flight as there are less delays on a train. Planes in China are very congested due to air traffic congestion.
Train from Hangzhou to Yiwu
You can also take a train from Hangzhou - this is a common one and many do this as there are a ton of flights to and from Hangzhou throughout the day.
Here’s a sample of Hangzhou trains going to Yiwu
You can find flights into Yiwu airport - but it will cost more than Hangzhou city and there are less choices.
Many fly into Hangzhou and then take one of the many trains into Yiwu market.
Here’s a sample of flights from Shenzhen to Hangzhou
Airasia has direct flights on Wednesdays.
You can take a bus - there is a huge bus network in China. Check in your city - it is a common one from Shanghai and other cities throughout Zhejiang province.