I have traveled to China about 10 times now as my wife was born there.
If you are not spending your time in air-conditioned hotels and doing some independent travel I seriously suggest you take some repellant.
It really depends in China where you are as to what sort of things you can buy; cities are expanding all the time; if you are in a newer section of city or an area that has a lot of expats then you can easily buy things like repellant, deodorant or shaving cream. However, if you are in an older city trying to hunt these things down is like hens teeth.
What the locals use is a product which is like a floral water. The most popular brand is Liu Shen 六神, which you will find in any convenience / grocery store and is easily recognisable as a bright green liquid in a slender glass bottle. The product in China is referred to as Hua Lu Shui 花露水. You can now also get this in a plastic bottle as a spray on.
This stuff is usually used quite liberally, so don't try and just use a few drops. Chinese medicinal type stuff is usually not as powerful as Western style so you need to use more.
The other thing you need to know, if you are not staying inside a decent quality air-conditioned hotel you should use a mosquito net.
My son and wife are preferred targets for mosquitoes in China so this is always something we always need to worry about.