There are many factories in Dongguan.

Shenzhen!
There are a good many factory outlets and distributors in the Huaqiangbei Commercial Zone - you can take the subway to get there. There are also several hotels in the area.. BUT be advised there are several large buildings to search thru. SEG does have a help desk to assist you in that building.
For large quantities find the factory outlet booths - you have to hunt.
If you need a guide, QA, interpreter, shipper let me know. I can recommend good people.