Step Down transformers are very easy to source in China. Most of them are made there anyway. Over 80 million Chinese travel Internationally each year for business, pleasure and education and many people would know about these devices.
The voltage in China is 220V / 50 Hz, except for Hong Kong where it is 200V / 50 Hz. You will find the society much more advanced than you can imagine. For example starting about two years ago China manufactured and sold internally 3 million more cars than USA. More Chinese people can afford to buy a car each year than Americans. They have that extensive 300+ km/h train system. It is easy there to handle technical issues.
As far as your amps I wouldn't bother converting the amp power transformers to an International type unless you plan to live in China permanently. Step Down transformers work just fine. You plug them into the wall socket and they will have American sockets on the step down transformer. All you need to work out is the wattage you need, as they come in different wattage capacities. I've used this method with many tube amps. I wouldn't worry about your amps as this method is reliable and safe.
Try and source step down transformers capable of 120V output with 220V source. Many 220V transformers output at 110V instead of 120V. It is best for the amps to have 120V output.
Walmart power packs aren't rated at high enough wattage to work with a tube amp. The heavy duty step down transformers are the right type to get. They shouldn't be expensive in China.